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Versions: 00 01 02 03 04 05 06 RFC 1970

INTERNET-DRAFT                                        Thomas Narten, IBM
July 7, 1995                             Erik Nordmark, Sun Microsystems
                                                 W A Simpson, Daydreamer



               Neighbor Discovery for IP Version 6 (IPv6)

                  <draft-ietf-ipngwg-discovery-01.txt>


Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   To learn the current status of any Internet-Draft, please check the
   ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow
   Directories on ds.internic.net (US East Coast), nic.nordu.net
   (Europe), ftp.isi.edu (US West Coast), or munnari.oz.au (Pacific
   Rim).

   Distribution of this memo is unlimited.

   This Internet Draft expires January 7, 1996.



Abstract

   This document specifies the Neighbor Discovery protocol for the IP
   Version 6 protocol.  IPv6 nodes on the same link use Neighbor
   Discovery to discover each other's presence, to determine each
   other's link-layer addresses, to find routers and to maintain
   reachability information about the paths to active neighbors.









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Contents

   Status of this Memo.......................................    1

   1.  INTRODUCTION..........................................    3

   2.  TERMINOLOGY...........................................    4

   3.  PROTOCOL OVERVIEW.....................................    8
      3.1.  Comparison with IPv4.............................   12
      3.2.  Supported Link Types.............................   13

   4.  CONCEPTUAL MODEL OF A HOST............................   14
      4.1.  Conceptual Data Structures.......................   15
      4.2.  Conceptual Sending Algorithm.....................   16
      4.3.  Garbage Collection and Timeout Requirements......   17

   5.  ROUTER AND PREFIX DISCOVERY...........................   18
      5.1.  Message Formats..................................   19
         5.1.1.  Router Solicitation Message Format..........   19
         5.1.2.  Router Advertisement Message Format.........   20
      5.2.  Router Specification.............................   22
         5.2.1.  Router Configuration Variables..............   22
         5.2.2.  Message Validation by Routers...............   25
         5.2.3.  Router Behavior.............................   26
         5.2.4.  Designated Addresses........................   30
      5.3.  Host Specification...............................   31
         5.3.1.  Host Configuration Variables................   31
         5.3.2.  Host Variables..............................   31
         5.3.3.  Message Validation by Nodes.................   32
         5.3.4.  Host Behavior...............................   32

   6.  ADDRESS RESOLUTION AND NEIGHBOR UNREACHABILITY DETECTION   36
      6.1.  Message Formats..................................   36
         6.1.1.  Neighbor Solicitation Message Format........   36
         6.1.2.  Neighbor Advertisement Message Format.......   38
      6.2.  Address Resolution...............................   40
         6.2.1.  Message Validation by Nodes.................   40
         6.2.2.  Node Specification..........................   41
         6.2.3.  Sending Node Specification..................   41
         6.2.4.  Target Node Specification...................   43
         6.2.5.  Anticipated Link-Layer Address Changes......   44
         6.2.6.  Anycast Neighbor Advertisements.............   45
         6.2.7.  Proxy Neighbor Advertisements...............   45
      6.3.  Neighbor Unreachability Detection................   46
         6.3.1.  Reachability Confirmation...................   46
         6.3.2.  Node Behavior...............................   47




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   7.  REDIRECT FUNCTION.....................................   50
      7.1.  Redirect Message Format..........................   50
      7.2.  Router Specification.............................   51
      7.3.  Host Specification...............................   52
         7.3.1.  Message Validation by Hosts.................   53
         7.3.2.  Host Behavior...............................   53

   8.  OPTIONS...............................................   54
      8.1.  Source/Target Link-layer Address.................   56
      8.2.  Prefix Information...............................   57
      8.3.  Redirected Header................................   58
      8.4.  MTU..............................................   59

   9.  MULTIHOMED HOSTS......................................   60

   10.  PROTOCOL CONSTANTS...................................   62

   11.  SECURITY CONSIDERATIONS..............................   62

   REFERENCES................................................   64

   AUTHORS' ADDRESSES........................................   65

   CHANGES SINCE PREVIOUS DOCUMENT...........................   66






1.  INTRODUCTION

This specification defines the Neighbor Discovery (ND) protocol for the
IP Version 6 protocol.  Nodes (hosts and routers) use Neighbor Discovery
to determine the link-layer addresses for neighbors known to reside on
attached links and to quickly purge cached values that become invalid.
Hosts also use Neighbor Discovery to find neighboring routers that are
willing to forward packets on their behalf.  Finally, nodes use the
protocol to actively keep track of which neighbors are reachable and
which are not, and to detect changed link-layer addresses.  Sending
hosts also detect when routers fail and actively search for functioning
alternates.

This document is a revision of <draft-ietf-ipngwg-discovery-00.txt>
which was itself a revision of the protocol specified in the two
documents:
     <draft-simpson-ipv6-discov-formats-02.txt>, and
     <draft-simpson-ipv6-discov-process-02.txt>



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The authors would like to acknowledge the contributions the IPNGWG
working group an, in particular, (in alphabetical order) Ran Atkinson,
Jim Bound, Scott Bradner, Stephen Deering, Robert Hinden, Allison
Mankin, Dan McDonald, and Sue Thomson.


2.  TERMINOLOGY

   IP          - Internet Protocol Version 6.  The terms IPv4 and IPv6
                 are used only in contexts where necessary to avoid
                 ambiguity.

                 ICMP        - Internet Message Control Protocol for the
                 Internet Protocol Version 6.  The terms ICMPv4 and
                 ICMPv6 are used only in contexts where necessary to
                 avoid ambiguity.

   node        - a device that implements IP.

   router      - a node that forwards IP packets not explicitly
                 addressed to itself.

   host        - any node that is not a router.

   upper layer - a protocol layer immediately above IP.  Examples are
                 transport protocols such as TCP and UDP, control
                 protocols such as ICMP, routing protocols such as OSPF,
                 and internet or lower-layer protocols being "tunneled"
                 over (i.e., encapsulated in) IP such as IPX, AppleTalk,
                 or IP itself.

   link        - a communication facility or medium over which nodes can
                 communicate at the link layer, i.e., the layer
                 immediately below IP.  Examples are Ethernets (simple
                 or bridged); PPP links; X.25, Frame Relay, or ATM
                 networks; and internet (or higher) layer "tunnels",
                 such as tunnels over IPv4 or IPv6 itself.

   interface   - a node's attachment to a link.

   neighbors   - nodes attached to the same link.

   address     - an IP-layer identifier for an interface or a set of
                 interfaces.

   designated address
               - a per-interface address selected from the interface's
                 assigned addresses.  Only routers are required to have



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                 designated addresses, which are used as the source
                 address in certain Neighbor Discovery messages sent by
                 the router from the interface.  Neighboring nodes use
                 the designated address to uniquely identify a router's
                 interface, which might have many addresses assigned to
                 it.  The designated address should change only
                 infrequently and must be a link-local address.

   anycast address
               - an identifier for a set of interfaces (typically
                 belonging to different nodes).  A packet sent to an
                 anycast address is delivered to one of the interfaces
                 identified by that address (the "nearest" one,
                 according to the routing protocols' measure of
                 distance).  See [ADDR-ARCH].

   link-layer address
               - a link-layer identifier for an interface.  Examples
                 include IEEE 802 addresses for Ethernet links and E.164
                 addresses for ISDN links.

   on-link     - an address that is assigned to a neighbor's interface
                 on a shared link.  A host considers an address to be
                 on-link if:
                   - it is covered by one of the link's prefixes, or
                   - a neighboring router specifies the address as the
                     target of a Redirect message, or
                   - a Neighbor Advertisement message is received for
                     the target address, or
                   - a Router Advertisement message is received from the
                     address.

   off-link    - the opposite of "on-link"; an address that is not
                 assigned to any interfaces attached to a shared link.

   reachability
               - whether or not packets sent by an IP node are properly
                 reaching a neighboring node.  For routers, reachability
                 means that packets sent by a node's IP layer are
                 delivered to the router's IP layer, and the router is
                 indeed forwarding packets (i.e., it is configured as a
                 router, not a host).  For hosts, reachability means
                 that packets sent by a node's IP layer are delivered to
                 the neighbor host's IP layer.  Note that reachability
                 only applies to the one-way "forward" path from a node
                 to a neighbor.

   packet      - an IP header plus payload.



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   link MTU    - the maximum transmission unit, i.e., maximum packet
                 size in octets, that can be conveyed in one piece over
                 a link.

   target      - an address about which address resolution information
                 is sought, or an address which is the new first-hop
                 when being redirected.

   proxy       - a router that responds to Neighbor Discovery query
                 messages on behalf of another node.  A router acting on
                 behalf of a mobile node that has moved off-link
                 potentially acts as a proxy for the mobile node.

   ICMP destination unreachable indication
               - an error indication returned to the original sender of
                 a packet that cannot be delivered for the reasons
                 outlined in [ICMPv6].  If the error occurs on a node
                 other than the node originating the packet, an ICMP
                 error message is generated.  If the error occurs on the
                 originating node, an implementation is not required to
                 actually create and send an ICMP error packet to the
                 source, as long as the sender is notified through an
                 appropriate mechanism (e.g., return value from a
                 procedure call).  Note, however, that an implementation
                 may find it convenient in some cases to return errors
                 to the sender by taking the offending packet,
                 generating an ICMP error message, and then delivering
                 it (locally) through the generic error handling
                 routines.

Different link layers have different properties.  The ones of concern to
Neighbor Discovery are:

   point-to-point
                  - a link that connects exactly two interfaces.

   multicast      - a link that supports some mechanism at the link
                    layer for sending packets to all (i.e. broadcast) or
                    a subset of all neighbors.  Multicast/broadcast can
                    be provided by a variety of link layer mechanisms
                    such as the physical link layer itself (for example,
                    Ethernet), replicated unicast packets sent by the
                    link layer software, or multicast servers (such as
                    in ATM).  Note that all point-to-point links are
                    multicast links.

   non-broadcast multi-access (NBMA)
                  - a link with more than two neighbors that does not



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                    support any form of multicast or broadcast (e.g.,
                    Frame Relay).

   shared media   - a link that allows direct communication among a
                    number of nodes, but attached nodes are configured
                    in such a way that they do not complete prefix
                    information about all on-link destinations.
                    Examples are large (switched) public data networks
                    such as SMDS and B-ISDN.  Also known as "large
                    clouds".  See [SH-MEDIA].

   variable MTU   - a link that does not have a well-defined MTU.  For
                    example, Token Ring (IEEE 802.5).  Other links like
                    Ethernet have a standard MTU defined by the link-
                    layer protocol.

   asymmetric reachability
                  - a link where non-reflexive and/or non-transitive
                    reachability is part of normal operation.  (Non-
                    reflexive reachability means packets from A reach B
                    but packets from B don't reach A.  Non-transitive
                    reachability means packets from A reach B, and
                    packets from B reach C, but packets from A don't
                    reach C.)  Many radio links exhibit these
                    properties.


Neighbor Discovery makes use of a number of different addresses defined
in [ADDR-ARCH], including:

   all-nodes multicast address
               - the link scope address to reach all nodes.  FF02::1

   all-routers multicast address
               - the link scope address to reach all routers.  FF02::2

   solicited-node multicast address
               - a multicast address that is computed as a function of
                 the solicited target's address.  The solicited-node
                 multicast address is formed by taking the low-order 32
                 bits of the IP address and appending those bits to the
                 96-bit prefix FF02:0:0:0:0:1, resulting in a multicast
                 address in the range FF02::1:0:0 to FF02::1:FFFF:FFFF.
                 For example, the solicited node multicast address
                 corresponding to the IP address 4037::01:800:200E:8C6C
                 is FF02::1:200E:8C6C.  IP addresses that differ only in
                 the high-order bits, e.g. due to multiple high-order
                 prefixes associated with different providers, will map



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                 to the same solicited-node address thereby reducing the
                 number of multicast addresses a node must join.

   unspecified address
               - the address 0:0:0:0:0:0:0:0.  It indicates the absence
                 of an address.  One example of its use is in the Source
                 Address field of Neighbor Solicitation messages sent by
                 an initializing host verifying that an address is
                 unique (e.g., no other node is already using the
                 address) [ADDRCONF].



3.  PROTOCOL OVERVIEW

This protocol solves a set of problems related to the interaction
between nodes attached to the same link.  It defines mechanisms for
solving each of the following problems:

    Router Discovery: How hosts locate routers that reside on an
               attached link.

    Prefix Discovery: How hosts discover the set of address prefixes
               that define which destinations are on-link for an
               attached link.  (Nodes use prefixes to distinguish
               destinations that reside on-link from those only
               reachable through a router.)

    Parameter Discovery: How a node learns such link parameters as the
               link MTU or such Internet parameters as the maximum hop
               limit value to place in outgoing packets, etc.

    Address Autoconfiguration: How nodes automatically configure an
               address for an interface.

    Address Resolution: How nodes determine the link-layer address of a
               neighboring node given only the node's IP address.

    Next-hop determination: The algorithm for mapping an IP destination
               address into the IP address of the neighbor to which
               traffic for the destination should be sent.  The next-hop
               can be a router or the destination itself.

    Neighbor Unreachability Detection: How nodes determine that a
               neighbor is no longer reachable.  For neighbors used as
               routers, alternate default routers can be tried.  For
               both routers and hosts, address resolution can be
               performed again.



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    Duplicate Address Detection: How a node detects if another node has
               been configured to use the same IP address.

    Redirect:  How a router informs a host of a better first-hop node to
               reach a particular destination.


Neighbor Discovery defines five different ICMP packet types: A pair of
Router Solicitation and Router Advertisement messages, a pair of
Neighbor Solicitation and Neighbor Advertisements messages, and a
Redirect message.  The messages serve the following purpose:

    Router Solicitation: When an interface becomes enabled, hosts may
               send out Router Solicitations that request routers to
               generate Router Advertisements immediately rather than at
               their next scheduled time.

    Router Advertisement: Routers advertise their presence together with
               various link and Internet parameters either periodically,
               or in response to an explicit Router Solicitation
               message.  Router Advertisements contain prefixes that are
               used for on-link determination and/or address
               configuration, a Maximum Hop Limit value, etc.

    Neighbor Solicitation: Sent by a node to determine the link-layer
               address of a neighbor, or to verify that a neighbor is
               still reachable via a cached link-layer address.
               Neighbor Solicitations are also used for Duplicate
               Address Detection.

    Neighbor Advertisement: A response to a Neighbor Solicitation
               message.  A node may also send unsolicited Neighbor
               Advertisements to announce a link-layer address change.

    Redirect:  Used by routers to inform hosts of a better first hop for
               a destination.

On multicast-capable links, each router periodically multicasts a Router
Advertisement packet announcing its availability.  A host receives
Router Advertisements from all routers, building a list of default
routers.  Routers generate Router Advertisements frequently enough that
hosts will learn of their presence within a few minutes, but not
frequently enough to rely on an absence of advertisements to detect
router failure; a separate Neighbor Unreachability Detection algorithm
handles this condition.

Router Advertisements contain a list of prefixes that can be used for
on-link determination and/or autonomous address configuration; flags in



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the prefixes specify the intended uses of a particular prefix.  Hosts
use the advertised on-link prefixes to build and maintain a list that is
used in deciding when a packet's destination is on-link or beyond a
router.  Note that a destination can be on-link even though it is not
covered by any advertised on-link prefix.  In such cases a router can
send a Redirect informing the sender that the destination is a neighbor.

Router Advertisements (and per-Prefix flags) allow routers to inform
hosts how to perform Address Autoconfiguration.  For examples, routers
can specify whether hosts should use stateful (DHCPv6) or autonomous
(stateless) address configuration.  Router Advertisement messages also
specify lifetimes for addresses that are configured using autonomous
address configuration.  The exact semantics and usage of the address
configuration-related information is specified in [ADDRCONF].

Router Advertisement messages also contain Internet parameters such as
the maximum hop that hosts should use in outgoing packets and,
optionally, link parameters such as the link MTU.  This facilitates
centralized administration of critical parameters that can be set on
routers and automatically propagated to all attached hosts.

Nodes accomplish Address Resolution by multicasting a Neighbor
Solicitation that asks the target node to return its link-layer address.
Neighbor Solicitation messages are multicast to the solicited-node
multicast address corresponding to the target address.  The target
returns its link-layer address in a unicast Neighbor Advertisement
message.  A single request-response pair of packets is sufficient for
both the initiator and the target to resolve each other's link-layer
addresses; the initiator includes its link-layer address in the Neighbor
Solicitation.

Neighbor Solicitation messages can also be used to determine if more
than one node has been configured to use a particular unicast address.
The use of Neighbor Solicitation messages for Duplicate Address
Detection is specified in [ADDRCONF].

Neighbor Unreachability Detection detects both the failure of a neighbor
or the failure of the forward path to the neighbor.  Doing so requires
positive confirmation that packets sent to a neighbor are actually
reaching that neighbor and being processed properly by its IP layer.
Neighbor Unreachability Detection uses confirmation from two sources.
When possible, upper-layer protocols provide a positive confirmation
that a connection is making "forward progress", that is, previously sent
data is known to have been delivered correctly (e.g., new
acknowledgments were received recently).  When positive confirmation is
not forthcoming through such "hints", a node sends explicit unicast
Neighbor Solicitation messages that solicit Neighbor Advertisements as
reachability confirmation from the next hop.  To reduce unnecessary



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network traffic, probe messages are only sent to neighbors to which the
node is actively sending packets.

In addition to addressing the above general problems, Neighbor Discovery
also handles the following situations:

     Link-layer address change - A node that knows its link-layer
          address has changed can multicast a few (unsolicited) Neighbor
          Advertisement packets to all nodes to quickly (but unreliably)
          update cached link-layer addresses that have become invalid.
          The Neighbor Unreachability Detection algorithm ensures that
          all nodes will reliably discover the new address, though the
          delay may be somewhat longer.

     Inbound load balancing - Nodes with replicated interfaces may want
          to load balance the reception of incoming packets across
          multiple network interfaces on the same link.  Such nodes have
          multiple link-layer addresses assigned to the same interface.
          For example, a single network driver could represent multiple
          network interface cards as a single logical interface having
          multiple link-layer addresses.  Load balancing is handled by
          allowing routers to omit the source link-layer address from
          Router Advertisement packets, thereby forcing neighbors to use
          Neighbor Solicitation messages to learn the link-layer
          addresses.  Returned Neighbor Advertisement messages can then
          contain different link-layer addresses dependent on who issued
          the solicitation.

     Anycast addresses - Anycast addresses identify one of a set of
          nodes providing an equivalent service, and multiple nodes on
          the same link may be configured to recognize the same Anycast
          address.  Neighbor Discovery handles the case when a node
          determines that an anycast address is on-link and sends a
          Neighbor Solicitation.  The potentially multiple Neighbor
          Advertisements for the anycast address will be identified as
          anycast/proxy responses.  When multiple such advertisements
          are received, rules specify precedence and how to break ties.

     Proxy advertisements - A router willing to accept packets on behalf
          of a target address that is unable to respond to Neighbor
          Solicitations can issue proxy Neighbor Advertisements.  There
          is currently no specified use of proxy, but proxy advertising
          could potentially be used to handle cases like mobile nodes
          that have moved off-link.  However, it is not intended as a
          general mechanism to handle nodes that, e.g., do not implement
          this protocol.  Proxy advertisements invoke the same
          precedence and tie-breaking rules as does Anycast.




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3.1.  Comparison with IPv4

The IPv6 Neighbor Discovery protocol corresponds to a combination of the
IPv4 protocols ARP [ARP], ICMP Router Discovery [RDISC], and ICMP
Redirect [ICMPv4].  In IPv4 there is no generally agreed upon protocol
or mechanism for Neighbor Unreachability Detection, although the Hosts
Requirements [HR-CL] does specify some possible algorithms for Dead
Gateway Detection (which address only a subset of the problems Neighbor
Unreachability Detection tackles).

The Neighbor Discovery protocol provides a multitude of improvements
over the IPv4 set of protocols:

     Router Discovery is part of the base protocol set; there is no need
     for hosts to "snoop" the routing protocols.

     Router advertisements carry link-layer addresses; no additional
     packet exchange is needed to resolve the router's link-layer
     address.

     Router advertisements carry prefixes for a link; there is no need
     to have a separate mechanism to configure the "netmask".

     Router advertisements contain hooks for Address Autoconfiguration.

     By default, hosts learn all on-link prefixes from Router
     Advertisements.  However, routers may be configured to omit some or
     all prefixes from Router Advertisements.  In such cases hosts will
     assume that destinations are off-link and send traffic to routers
     by default.  A router can then issue redirects for on-link
     destinations as appropriate.  This mechanism may be useful on
     shared media links where it is undesirable or not possible for
     nodes to know all prefixes for on-link destinations.

     Routers can advertise an MTU for hosts to use on the link, ensuring
     that all nodes use the same MTU value on links lacking a well-
     defined MTU.

     Address Resolution uses multicast "spread" over 4 billion (2^32)
     multicast addresses resulting in greatly reduced Address Resolution
     related interrupts for nodes other than the target and generates no
     interrupts on non-IPv6 nodes.

     Redirects contain the link-layer address of the new first hop;
     separate Address Resolution is not needed upon receiving a
     redirect.

     Nodes assume that the new next-hop target address in a Redirect is



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     on-link making it possible to redirect to targets that do not share
     a common address prefix with the sender.  This is an implementation
     of the XRedirect idea in [SH-MEDIA], which simplifies some aspects
     of neighbor interaction on shared media.

     Neighbor Unreachability Detection is part of the base,
     significantly improving the robustness of packet delivery in the
     presence of failing routers, partially failing or partitioned links
     and nodes that change their link-layer addresses.  For instance,
     mobile nodes can move off-link without loosing any connectivity due
     to stale ARP caches.

     Unlike ARP, Neighbor Discovery detects half-link failures and tries
     to avoid using neighbors with which there is not two-way
     connectivity.

     Placing address resolution at the ICMP layer makes the protocol
     more media-independent than ARP and makes it possible to use
     standard IP authentication and security mechanisms as appropriate
     [IPv6-AUTH, IPv6-ESP].



3.2.  Supported Link Types

Neighbor Discovery supports links with different properties.  In the
presence of certain properties only a subset of the ND protocol is
available:

   point-to-point - Neighbor Discovery handles such links just like
                    multicast links.  (Multicast can be trivially
                    provided on point to point links, and interfaces can
                    be assigned link-local addresses.)


   multicast      - All aspects of Neighbor Discovery are available.

   non-broadcast multiple access (NBMA)
                  - The only Neighbor Discovery mechanisms available on
                    these links are Redirect handling and Neighbor
                    Unreachability Detection.

                    If the hosts support manual configuration of a list
                    of default routers the hosts can dynamically acquire
                    the link-layer addresses for their neighbors from
                    Redirect messages.

   shared media   - The Redirect message is modeled after the XRedirect



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                    message in [SH-MEDIA] in order to simplify use of
                    the protocol on shared media links.

                    This specification does not address shared media
                    issues that only relate to routers, such as:

                     - How routers exchange reachability information on
                       a shared media link.

                     - How a router determines the link-layer address of
                       a host, which it needs to send redirect messages
                       to the host.

                     - How a router determines that it is the first hop
                       router for a received packet.


                    The protocol is extensible (through the definition
                    of new options) so that other solutions might be
                    possible in the future.

   variable MTU   - Neighbor Discovery allows the routers to specify a
                    MTU for the link.  This allows all nodes to use the
                    same MTU.  Note: It is not possible to have each
                    node use a different MTU (or Maximum Receive Unit)
                    due to multicast.

   asymmetric reachability
                  - Neighbor Discovery detects the absence of symmetric
                    reachability; a node avoids using a neighbor with
                    which it does not have symmetric connectivity.

                    The protocol can presumably be extended in the
                    future to find viable paths in environments that
                    lack reflexive and transitive connectivity.


4.  CONCEPTUAL MODEL OF A HOST

This section describes a conceptual model of one possible data structure
organization that hosts (and to some extent routers) will maintain in
interacting with neighboring nodes.  The described organization is
provided to facilitate the explanation of how the Neighbor Discovery
protocol should behave.  This document does not mandate that
implementations adhere to this model as long as their behavior is
consistent with the protocol specification.

This model is only concerned with the aspects of host behavior directly



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related to Neighbor Discovery.  In particular, it does not concern
itself with such issues as source address selection or the selecting of
an outgoing interface on a multihomed host.


4.1.  Conceptual Data Structures

Hosts will need to maintain the following pieces of information about an
interface:

   Neighbor Cache - A set of entries about individual neighbors to which
                    traffic has been sent recently.  Entries are keyed
                    on the neighbor's IP address and contain such
                    information as its link-layer address, a flag
                    indicating whether the neighbor is a router or a
                    host (called "is_router" in this document), a
                    pointer to any queued packets waiting for Address
                    Resolution to complete, etc.

                    A Neighbor Cache entry also contains information
                    used by the Neighbor Unreachability Detection
                    algorithm.  This includes the reachability state,
                    the number of unanswered probes, and the time the
                    next Neighbor Unreachability Detection event is
                    scheduled to take place.

   Destination Cache
                  - A set of entries for each destination to which
                    traffic has been sent recently.  The Destination
                    Cache includes both on-link and off-link
                    destinations and provides a level of indirection
                    into the Neighbor Cache; the Destination Cache maps
                    a destination IP address to the IP address of the
                    next-hop neighbor.  Implementations may find it
                    convenient to store additional information not
                    directly related to Neighbor Discovery in
                    destination entries, such as the Path MTU (PMTU) and
                    round trip timers maintained by transport protocols.

   Prefix List    - A list of the prefixes that define a set of
                    addresses that are on-link.  Prefix list entries are
                    created from information received in Router
                    Advertisements.  Each entry has an associated
                    invalidation timer value (extracted from the
                    advertisement) used to delete prefixes that routers
                    stop advertising.  The invalidation timer can have a
                    value of infinity to make sure the prefix remains
                    valid indefinitely until it is explicitly advertised



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                    with a lower timer value.

   Default Router List
                  - A list of routers to which packets may be sent.
                    Router list entries point to entries in the Neighbor
                    Cache so that when a router is being selected,
                    routers known to be reachable can be favored over
                    those whose reachability is suspect.  Each entry
                    also has an associated invalidation timer value
                    (extracted from Router Advertisements) used to
                    delete entries that are no longer advertised.

Note that the above conceptual data structures can be implemented using
a variety of techniques.  One possible implementation is to use a single
longest-match routing table for all of the above data structures.


The Neighbor Cache contains information maintained by the Neighbor
Unreachability Detection algorithm.  A key piece of information is a
neighbor's reachability state which has three possible values:

   INCOMPLETE  Address Resolution is in progress and the link-layer
               address of the neighbor has not yet been determined.

   REACHABLE   Roughly speaking, the neighbor is known to have been
               reachable recently (within tens of seconds ago).

   PROBE       The neighbor is probably reachable, but the last explicit
               reachability confirmation was received long enough ago
               that verification is now actively sought.


4.2.  Conceptual Sending Algorithm

When sending a packet, a node uses a combination of the Destination
Cache, the Prefix List, and the Default Router List to determine the IP
address of the appropriate next hop, an operation known as "next-hop
determination".  Once the IP address of the next hop is known, the
Neighbor Cache is consulted for link-level information about that
neighbor.

Next-hop determination operates as follows.  The sender examines the
Prefix List to determine whether the packet's destination is on- or
off-link.  If the destination is on-link, the next-hop address is the
same as the packet's destination address.  If the destination is off-
link, the sender selects a router from the Default Router List
(following the rules described in Section 5.3.4).  If there are no
routers on the Default Router List, the sender assumes that the



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destination is on-link.

For efficiency reasons, next-hop determination is not performed on every
packet that is sent.  Instead, the results of next-hop determination
computations are saved in the Destination Cache.  When the sending node
has a packet to send, it first examines the Destination Cache.  If no
entry exists for the destination, next-hop determination is invoked to
create a Destination Cache entry.

Once the IP address of the next-hop node is known, the sender examines
the Neighbor Cache for link-level information about that neighbor.  If
no entry exists, the node creates an INCOMPLETE entry, sends a Neighbor
Solicitation message, and queues the packet pending completion of
Address Resolution.  When a Neighbor Advertisement response is received,
the link-layer addresses is entered in the Neighbor Cache entry and the
queued packet is transmitted.  The Address Resolution mechanism is
described in detail in Section 6.2.

Each time a Neighbor Cache entry is accessed while transmitting a
packet, the sender checks Neighbor Unreachability Detection related
information according to the Neighbor Unreachability Detection algorithm
(Section 6.3).  This check might result in the sender transmitting a
Neighbor Solicitation to verify that the neighbor is still reachable.

Next-hop determination is done the first time traffic is sent to a
destination.  As long as subsequent communication to that destination
proceeds successfully, the Destination Cache entry continues to be used.
If at some point communication ceases to proceed, as determined by the
Neighbor Unreachability Detection algorithm, next-hop determination may
need to be performed again.  For example, traffic through a failed
router should be switched to a working router.  Likewise, it may be
possible to reroute traffic destined for a mobile node to a "mobility
agent".

Note that when a node redoes next-hop determination there is no need to
discard the complete Destination Cache entry.  In fact, it is generally
beneficial to retain such cached information as the PMTU and round trip
timer values that may also be kept in the Destination Cache entry.


4.3.  Garbage Collection and Timeout Requirements

The conceptual data structures described above use different mechanisms
for discarding potentially stale, as well as unused, information.

From the perspective of correctness, there is no need to periodically
purge Destination and Neighbor Cache entries.  Although stale
information can potentially remain in the cache indefinitely, the



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Neighbor Unreachability Detection algorithm described in this document
ensures that stale information is purged quickly if it is actually being
used.

To limit the storage needed for the Destination and Neighbor Caches, a
node may need to garbage-collect old entries.  However, care must be
taken to insure that sufficient space is always present to hold the
working set of active entries.  A small cache may result in an excessive
number of Neighbor Discovery messages as discarded entries are discarded
and rebuilt in quick succession.  Any LRU-based policy that only
reclaims entries that have not been used in some time (e.g, ten minutes
or more) should be adequate for garbage-collecting unused entries.

A node should retain entries in the Default Router List and the Prefix
List until their lifetimes expire.  However, a node may garbage collect
entries prematurely if it is low on memory.  If not all routers are kept
on the Default Router list, a node should retain at least two entries in
the Default Router List (and preferably more) in order to maintain
robust connectivity for off-link destinations.

When removing an entry from the Default Router List or the Prefix List
there is no need to purge any entries from the Destination or Neighbor
Caches.  Neighbor Unreachability Detection will effectively purge any
entries in these caches that have become stale.



5.  ROUTER AND PREFIX DISCOVERY

This section describes message formats, router behavior and host
behavior related to the Router Discovery portion of Neighbor Discovery.
Router Discovery is used to locate neighboring routers as well as learn
prefixes and configuration parameters related to address
autoconfiguration.

Prefix Discovery provides a mechanism through which hosts learn of
ranges of IP addresses that reside on-link and thus can be reached
directly without going through a router.  Routers advertise a set of
prefixes that cover those IP addresses that are on-link.  Prefix
discovery is logically separate from Router Discovery.  In practice,
prefix information is included in options piggybacked on Router
Advertisement messages to reduce network traffic.

Address Autoconfiguration information is also logically separate from
Router Discovery.  To reduce network traffic, however, autoconfiguration
information is piggybacked on Router Discovery messages.  In fact, the
same prefixes can be advertised for on-link determination and address
autoconfiguration by specifying the appropriate flags in the Prefix



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Information options.  This document does not define how
autoconfiguration information is processed.  See [ADDRCONF] for details.


5.1.  Message Formats


5.1.1.  Router Solicitation Message Format

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                            Reserved                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options ...
     +-+-+-+-+-+-+-+-+-+-+-+-

IP Fields:

   Source Address
                  MUST be the link-local address belonging to the
                  interface from which this message is sent.

   Destination Address
                  The all-routers multicast address.

   Hop Count      1

   Authentication Header
                  If a security association exists between the sender
                  and the destination the sender SHOULD include this
                  header.

ICMP Fields:

   Type           133

   Code           0

   Checksum       The ICMP checksum.  See [ICMPv6].

   Reserved       This field is unused.  It MUST be initialized to zero
                  by the sender and ignored by the receiver.

Options:

   Source link-layer address
                  The link-layer address for the sender.  SHOULD be



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                  included on link layers that have addresses in order
                  for the router to be able to send a Router
                  Advertisement without having to perform address
                  resolution on the host's address.

   Future versions of this protocol may define new option types.
   Receivers MUST skip over and ignore any options they do not recognize
   and continue processing the message.


5.1.2.  Router Advertisement Message Format

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Max Hop Limit |M|O|  Reserved |       Router Lifetime         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                        Reachable Time                         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                     Reachable Retrans Timer                   |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options ...
     +-+-+-+-+-+-+-+-+-+-+-+-

IP Fields:

   Source Address
                  The interface's designated IP address.

   Destination Address
                  Either the Source Address of an invoking Router
                  Solicitation or the all-nodes multicast address.

   Hop Count      1

   Authentication Header
                  If a security association exists between the sender
                  and the destination the sender SHOULD include this
                  header.

ICMP Fields:

   Type           134

   Code           0

   Checksum       The ICMP checksum.  See [ICMPv6].




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   Max Hop Limit  8-bit unsigned integer.  The maximum hop limit that
                  the router suggests that hosts use when sending IP
                  packets.  A value of zero means unspecified.

   M              1-bit "Managed address configuration" flag.  Use the
                  administered (stateful) protocol for address
                  autoconfiguration in addition to any addresses
                  autoconfigured using stateless address
                  autoconfiguration.  The use of this flag is described
                  in [ADDRCONF].

   O              1-bit "Other configuration" flag.  Use the
                  administered (stateful) protocol for autoconfiguration
                  of other (non-address) information.  The use of this
                  flag is described in [ADDRCONF].

   Reserved       A 6-bit unused field.  It MUST be initialized to zero
                  by the sender and ignored by the receiver.

   Router Lifetime
                  16-bit unsigned integer.  The lifetime associated with
                  the default router in units of seconds.  The maximum
                  value corresponds to 18.2 hours.  This lifetime does
                  not apply to information contained in any options in
                  the message.  Options that need time limits for their
                  information include their own lifetime fields.

   Reachable Time 32-bit unsigned integer.  The time, in milliseconds,
                  that a node considers a neighbor being reachable after
                  receiving some reachability confirmation.  Used by the
                  Neighbor Unreachability Detection algorithm.  A value
                  of zero means unspecified.

   Reachable Retrans Timer
                  32-bit unsigned integer.  The time, in milliseconds,
                  between retransmitted Neighbor Solicitation probes to
                  a neighbor that is not returning solicited Neighbor
                  Advertisements.  Used by the Neighbor Unreachability
                  Detection algorithm.  A value of zero means
                  unspecified.

Options:

   Source link-layer address
                  The link-layer address for the router.  Only used on
                  link layers that have addresses.  A router MAY omit
                  this option in order to enable inbound load sharing
                  across multiple link-layer addresses.



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   MTU            SHOULD be sent on links that have a variable MTU.  MAY
                  be sent on other links.

   Prefix Information
                  A router MAY include 0 or more Prefix Information
                  options.  These options specify the prefixes that are
                  on-link and/or are used for address autoconfiguration.
                  A router SHOULD include all on-link prefixes on
                  multicast links.  This enables multihomed hosts to do
                  optimal outgoing interface selection for neighboring
                  nodes.


   Future versions of this protocol may define new option types.
   Receivers MUST skip over and ignore any options they do not recognize
   and continue processing the message.


5.2.  Router Specification


5.2.1.  Router Configuration Variables

A router MUST allow for the following variables to be configured by
system management; default values are specified so as to make it
unnecessary to configure any of these variables in many cases.

For each multicast interface:
     AdvertiseDefault
                    A flag indicating whether or not the router should
                    advertise itself as a default router on the
                    interface.

                    Default: TRUE

     ManagedFlag
                    The true/false value to be placed in the "Managed
                    address configuration" field in the Router
                    Advertisement.  See [ADDRCONF].

                    Default: FALSE

     OtherFlag      The true/false value to be placed in the "Other
                    configuration" field in the Router Advertisement.
                    See [ADDRCONF].

                    Default: FALSE




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     DesignatedAddress
                    The address to be used as the source address in
                    certain Neighbor Discovery messages sent on the
                    interface.

                    Default: The interface's link-local address.

     LinkMTU        The value to be placed in MTU options sent by the
                    router.  If the value is set to zero no MTU options
                    are sent.

                    Default: 0

     ReachableTime
                    The value to be placed in the Reachable Time field
                    in the Router Advertisement messages sent by the
                    router.  The value zero means unspecified (by this
                    router).

                    Default: REACHABLE_TIME milliseconds

     ReachableRetransTimer
                    The value to be placed in the Reachable Retrans
                    Timer field in the Router Advertisement messages
                    sent by the router.  The value zero means
                    unspecified (by this router).

                    Default: REACHABLE_RETRANS_TIMER milliseconds

     MaximumHopLimit
                    The value to be placed in the Max Hop Limit field in
                    the Router Advertisement messages sent by the
                    router.  The value zero means unspecified (by this
                    router).

                    Default: The value specified in the most recent
                    "Assigned Numbers" RFC [ASSIGNED].

     MaxRtrAdvInterval
                    The maximum time allowed between sending multicast
                    Router Advertisements from the interface, in
                    seconds.  MUST be no less than 1 second and no
                    greater than 1800 seconds.

                    Default: 600 seconds

     MinRtrAdvInterval
                    The minimum time allowed between sending unsolicited



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                    multicast Router Advertisements from the interface,
                    in seconds.  MUST be no less than 0.1 seconds and no
                    greater than MaxRtrAdvInterval.

                    Default: 0.33 * MaxRtrAdvInterval

     RtrAdvLifetime
                    The value to be placed in the Router Lifetime field
                    of Router Advertisements sent from the interface, in
                    seconds.  MUST be no less than MaxRtrAdvInterval and
                    no greater than 9000 seconds.  Note: if
                    AdvertiseDefault is false, the value of
                    RtrAdvLifetime is irrelevant; a Lifetime value of 0
                    in Router Advertisements indicates that the router
                    should not be used as a default router.

                    Default: 3 * MaxRtrAdvInterval

     PrefixList
                    A list of prefixes to be placed in Prefix
                    Information options in Router Advertisement messages
                    sent from the interface.

                    Default: The PrefixList contains all prefixes that
                    the router advertises via routing protocols as being
                    on the link on which the advertisement is sent.

                    Each prefix is associated with:

                       InvalidationLifetime
                            The value to be placed in the Invalidation
                            Lifetime in the Prefix Information option,
                            in seconds.  A designated value (like all-
                            one bits) can be used to represent infinity.

                            Default: infinity.

                       OnLinkFlag
                            The value to be placed in the on-link flag
                            ("L-bit") field in the Prefix Information
                            option.

                            Default: TRUE

                    Automatic address configuration [ADDRCONF] defines
                    additional information associated with each the
                    prefixes:




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                       DeprecationLifetime
                            The value to be placed in the Deprecation
                            Lifetime in the Prefix Information option,
                            in seconds.  A designated value (like all-
                            one bits) can be used to represent infinity.
                            See [ADDRCONF].

                            Default: 604800 seconds (7 days)

                       AutonomousFlag
                            The value to be placed in the Autonomous
                            Flag field in the Prefix Information option.
                            See [ADDRCONF].

                            Default: TRUE


Protocol constants are defined in Section 10.


5.2.2.  Message Validation by Routers

A router MUST silently discard any received Router Solicitation messages
that do not satisfy all of the following validity checks:

   - IP Source Address is a link-local address.

   - ICMP Checksum is valid.

   - ICMP Code is 0.

   - ICMP length (derived from the IP length) is 8 or more octets.

   - if the message includes an Authentication Header, the message
     authenticates correctly.

   - all included options have a length that is greater then zero.


The contents of the Reserved field, and of any unrecognized options,
MUST be ignored.  Future, backward-compatible changes to the protocol
may specify the contents of the Reserved field or add new options;
backward-incompatible changes may use different Code values.

A solicitation that passes the validity checks is called a "valid
solicitation".

Routers MUST also validate Router Advertisements as described in Section



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5.3.3.

Routers SHOULD receive Router Advertisements sent by other routers on
the link and verify that the routers are advertising consistent
information.  Detected inconsistencies indicate that one or more routers
might be misconfigured and SHOULD be logged to system or network
management.  The minimum set of information that should be checked:

 - Different Max Hop Limit values (except for the unspecified value of
   zero).

 - Different value of the M or O flags.

 - Different Reachable Time values (except for the unspecified value of
   zero).

 - Different Reachable Retrans Timer values (except for the unspecified
   value of zero).

 - Different values in the MTU options.

 - Different Invalidation Lifetime for the same prefix.

 - Different Deprecation Lifetime for the same prefix.


Note that it is expected that different routers advertise different sets
of prefixes.  Also, some routers might leave some fields as unspecified
i.e. with the value zero.

In addition, routers can optionally examine the source address of Router
Advertisements to determine which of a neighboring router's addresses is
its designated address.  Any other action on reception of Router
Advertisement messages by a router is beyond the scope of this document.


5.2.3.  Router Behavior

A router MUST join the all-routers multicast address on all multicast
capable interfaces.

The term "advertising interface" refers to any functioning and enabled
interface that has at least one IP address assigned to it.  From each
advertising interface, the router transmits periodic, multicast Router
Advertisements, containing the following values consistent with the
message format above:

   - In the Router Lifetime field: the interface's configured



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     RtrAdvLifetime.  If the router's AdvertiseDefault flag is set to
     false, the Router Lifetime field MUST be set to 0.

   - In the M and O flags: the interface's configured ManagedFlag and
     OtherFlag, respectively.  See [ADDRCONF].

   - In the Max Hop Limit field: the interface's configured
     MaximumHopLimit.

   - In the Reachable Time field: the interface's configured
     ReachableTime.

   - In the Reachable Retrans Timer field: the interface's configured
     ReachableRetransTimer.

   - In the options:

     Source Link-Layer Address option: link-layer address of the sending
          interface.  This option MAY be omitted to facilitate in-bound
          load balancing over replicated interfaces.

     Prefix Information options: one Prefix Information option for each
          prefix listed in PrefixList with the option fields set from
          the information in the PrefixList entry as follows:
             - In the "on-link" flag: the entry's OnLinkFlag.

             - In the Invalidation Lifetime field: the entry's
               InvalidationLifetime.  If the InvalidationLifetime is
               infinite the field is set to all one bits.

             - In the "Autonomous address configuration" flag: the
               entry's AutonomousFlag.

             - In the Deprecation Lifetime field: the entry's
               DeprecationLifetime.  If the DeprecationLifetime is
               infinite the field is set to all one bits.

The advertisements are not strictly periodic: the interval between
subsequent transmissions is randomized to reduce the probability of
synchronization with the advertisements from other routers on the same
link [SYNC].  This is done by maintaining a separate transmission
interval timer for each advertising interface.  Each time a multicast
advertisement is sent from an interface, that interface's timer is reset
to a uniformly-distributed random value between the interface's
configured MinRtrAdvInterval and MaxRtrAdvInterval; expiration of the
timer causes the next advertisement to be sent from the interface, and a
new random value to be chosen.  (It is recommended that routers include
some unique value, such as one of their IP or link-layer addresses, in



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the seed used to initialize their pseudo-random number generators.
Although the randomization range is configured in units of seconds, the
actual randomly-chosen values SHOULD not be in units of whole seconds,
but rather in units of the highest available timer resolution.)

For the first few advertisements sent from an interface (up to
MAX_INITIAL_RTR_ADVERTISEMENTS), if the randomly chosen interval is
greater than MAX_INITIAL_RTR_ADVERT_INTERVAL, the timer SHOULD be set to
MAX_INITIAL_RTR_ADVERT_INTERVAL instead.  Using this smaller interval
for the initial advertisements increases the likelihood of a router
being discovered quickly when it first becomes available, in the
presence of possible packet loss.

In addition to the periodic, unsolicited advertisements, a router sends
advertisements in response to valid solicitations received on any of its
advertising interfaces.  A router MAY choose to unicast the response
directly to the soliciting host's address, or multicast it to the all-
nodes address; in the latter case, the interface's interval timer is
reset to a new random value, as with unsolicited advertisements.  A
unicast response MAY be delayed, and a multicast response MUST be
delayed, for a small random interval not greater than
MAX_RTR_RESPONSE_DELAY, in order to prevent synchronization with other
responding routers, and to allow multiple, closely-spaced solicitations
to be answered with a single multicast advertisement.  A router that
chooses to delay responses can operate as follows:

 - When the router receiver a Router Solicitation it starts a timer with
   the above small random interval.

 - If the router receives an additional Router Solicitation on the
   interface while the timer is running it will multicast the response.
   Otherwise it will unicast the response.

 - The router sends the (multicast or unicast) Router Advertisement when
   the timer expires.  If the advertisement is multicast the router
   resets the interface's interval timer to a new random value, as with
   unsolicited advertisements.

Note that a router is permitted to send multicast Router Advertisements
more frequently than indicated by the MinRtrAdvInterval configuration
variable if the additional advertisements are responses to explicit
solicitations.  In all cases, however, unsolicited multicast
advertisements must not be sent more frequently than indicated by
MinRtrAdvInterval.

When a router receives a Router Solicitation it records the source of
the packet as being a neighbor.  If the solicitation contains a Source
Link-Layer Address option, and the router has a Neighbor Cache entry for



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the neighbor, the link-layer address should be updated in the Neighbor
Cache and the entry's "is_router" flag should be set to false.  If a
Neighbor Cache entry is created for the source its reachability state
MUST be set to PROBE as specified in Section 6.3.2.

It should be noted that an interface may become an advertising interface
at times other than system startup, as a result of recovery from an
interface failure or through actions of system management such as:

   - enabling the interface, if it had been administratively disabled,
     and its AdvertiseDefault flag is TRUE, or

   - enabling IP forwarding capability (i.e., changing the system from
     being a host to being a router), when the interface's
     AdvertiseDefault flag is TRUE, or

   - changing the AdvertiseDefault flag from FALSE to TRUE.

In such cases the router MUST commence transmission of periodic
advertisements on the new advertising interface, limiting the first few
advertisements to intervals no greater than
MAX_INITIAL_RTR_ADVERT_INTERVAL.  In the case of a host becoming a
router, the system MUST also join the all-routers IP multicast group on
all interfaces on which the router supports IP multicast (whether or not
they are advertising interfaces).

An interface may also cease to be an advertising interface, through
actions of system management such as:

   - administratively disabling the interface, or

   - shutting down the system, or disabling the IP forwarding capability
     (i.e., changing the system from being a router to being a host), or

   - setting the AdvertiseDefault flag of the interface to FALSE.

In such cases the router SHOULD transmit a final multicast Router
Advertisement on the interface with a Router Lifetime field of zero.  In
the case of a router becoming a host, the system MUST also depart from
the all-routers IP multicast group on all interfaces on which the router
supports IP multicast (whether or not they had been advertising
interfaces).  In addition, the host MUST insure that subsequent Neighbor
Advertisement messages sent from the interface have the Router flag set
to zero.

The information advertised in Router Advertisements may change through
actions of system management.  For instance, the lifetime for the
advertised prefixes may be changes, or the advertised MTU may change.



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In such cases, the router MAY transmit a few (no more than
MAX_INITIAL_RTR_AVERTISEMENTS) Router Advertisements separated by an
interval of MAX_INITIAL_RTR_ADVERT_INTERVAL.

A router might want to send Router Advertisements without advertising
itself as being a default router.  For instance, a router might
advertise prefixes for address autoconfiguration while not wishing to
forward packets.  Such a router MUST set the Router Lifetime field to
zero in its advertisements.

A router MAY choose to not include all Prefix Information options in
every Router Advertisement, if the prefix lifetimes are much longer than
RtrAdvLifetime.  However, when responding to a Router Solicitation the
router SHOULD transmit all prefixes to allow hosts to quickly discover
the prefixes during system initialization.


5.2.4.  Designated Addresses

Routers should take some care in selecting their designated address and
in handling any, hopefully infrequent, change of their designated
address.

The designated address SHOULD be one that changes infrequently over
time.  Nodes receiving Neighbor Discovery messages use the source
address to identify the sender.  If multiple packets from the same
neighbor contain different source addresses, nodes will assume they come
from different nodes, leading to undesirable behavior.  For example, a
node will ignore Redirect messages that are believed to have been sent
by a router other than the current first-hop router.

The designated address MUST be a link-local address; the link-local
address does not change when a site renumbers.

If a router changes the designated address for one of its interfaces, it
SHOULD inform hosts of this change.  The router should multicast a few
Router Advertisements with Router Lifetime field set to zero for the old
designated address and also multicast a few Router Advertisements for
the new designated address.  The exact procedures SHOULD be the same as
when an interface ceases being an advertising interface, and when an
interface becomes an advertising interface, respectively.

A router MUST be able to determine the designated address for each of
its neighboring routers in order to ensure that the target address in a
Redirect message identifies the neighbor router by its designated
address.  This may require that routing protocols exchange designated
addresses.




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5.3.  Host Specification


5.3.1.  Host Configuration Variables

None.


5.3.2.  Host Variables

A host maintains certain Neighbor Discovery related variables in
addition to the data structures defined in Section 4.1.  These variables
have default values that are overridden by information received in
Router Advertisement messages.  The default values are used when there
is no router on the link, or when all received Router Advertisements
have left the value unspecified.

For each interface:

     LinkMTU        The MTU of the link.

                    Default: The valued defined in the specific document
                    that describe how IPv6 operates over the particular
                    link layer.

     MaximumHopLimit
                    The maximum Hop Count to be used when sending IP
                    packets.

                    Default: The value specified in the most recent
                    "Assigned Numbers" RFC [ASSIGNED].

     ReachableTime
                    The time a neighbor is considered reachable after
                    receiving a reachability confirmation.

                    Default: REACHABLE_TIME milliseconds

     ReachableRetransTimer
                    The time between transmissions of Neighbor
                    Solicitation probes to a neighbor when verifying the
                    reachability of the neighbor.

                    Default: REACHABLE_RETRANS_TIMER milliseconds







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5.3.3.  Message Validation by Nodes

A node MUST silently discard any received Router Advertisement messages
that do not satisfy all of the following validity checks:

   - IP Source Address is a link-local address.

   - ICMP Checksum is valid.

   - ICMP Code is 0.

   - ICMP length (derived from the IP length) is 16 or more octets.

   - if the message includes an Authentication Header, the message
     authenticates correctly.

   - all included options have a length that is greater then zero.


The contents of the Reserved field, and of any unrecognized options,
MUST be ignored.  Future, backward-compatible changes to the protocol
may specify the contents of the Reserved field or add new options;
backward-incompatible changes may use different Code values.

An advertisement that passes the validity checks is called a "valid
advertisement".

A host MUST silently discard any received Router Solicitation messages.


5.3.4.  Host Behavior

The host joins the all-nodes multicast address on all multicast capable
interfaces.

A host MUST NOT send a Router Advertisement message at any time.

To process a valid Router Advertisement, a host extracts the source
address of the packet and does the following:

   - If the address is not already present in the host's Default Router
     List, and the advertisement's Router Lifetime is non-zero, create a
     new entry in the list, and initialize its timer to value in the
     advertisement's Router Lifetime field.

   - If the address is already present in the host's Default Router List
     as a result of a previously-received advertisement, reset its timer
     to the Router Lifetime value in the newly-received advertisement.



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   - If the received Router Lifetime value is zero, time-out the entry
     immediately and remove it from the Default Router list.


The source address in the Router Advertisement might not be covered by
any of the entries in the host's Prefix List or any of the Prefix
Information options in the message; a host MUST accept all valid
advertisements independent of their source address.

If the received Max Hop Limit value is non-zero the host SHOULD set its
MaximumHopLimit variable to the received value.  Hosts use the last Max
Hop Limit value they have received; routers should be configured to
advertise identical values to avoid hosts switching between different
values.

The host SHOULD set its ReachableTime variable to the Reachable Time
field, if the received value is non-zero.  Likewise it SHOULD set the
ReachableRetransTimer to the Reachable Retrans Timer field, if the
received value is non-zero.  Hosts use the last values they have
received; routers should be configured to advertise identical values to
avoid hosts switching between different values as they receive
advertisements from different routers.

After extracting information from the fixed part of the Router
Advertisement message, the advertisement MUST be scanned for valid
options.  If the advertisement contains a source link-layer address
option the link-layer address MUST be recorded in the Neighbor Cache
entry for the router (creating an entry if necessary) and the
"is_router" flag in the Neighbor Cache entry MUST be set to true.  The
"is_route" flag is used by Neighbor Unreachability Detection to
determine when a router changes to being a host (i.e. no longer capable
of forwarding packets).  If a Neighbor Cache entry is created for the
router its reachability state MUST be set to PROBE as specified in
Section 6.3.2.

Received MTU options are handled as specified in Section 8.4.

For each Prefix Information option that has the "on-link" (L) flag set,
the host does the following:

   - If the prefix is not already present in the Prefix List, create a
     new entry for the prefix and initialize its invalidation timer to
     the Invalidation Lifetime value in the Prefix Information option.

   - If the prefix is already present in the host's Prefix List as the
     result of a previously-received advertisement, reset its
     invalidation timer to the Invalidation Lifetime value in the Prefix
     Information option.



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   - If the received Invalidation Lifetime value is zero, time-out the
     prefix immediately.

   Note: Implementations can choose to process the on-link aspects of
   the prefixes separately from the address autoconfiguration aspects of
   the prefixes by e.g. passing a copy of each valid Router
   Advertisement message to both an "on-link" and an "addrconf"
   function.  Each function can then operate on the prefixes that have
   the appropriate flag set.

Whenever the invalidation timer expires for a Prefix List entry, that
entry is discarded.  No existing Destination Cache entries are affected,
however.

Whenever a timer expires for an entry in the Default Router List, that
entry is discarded.  Any entries in the Destination Cache going through
that router will continue to be used.  Neighbor Unreachability Detection
will purge them if appropriate.

To limit the storage needed for the Default Router List, a host MAY
choose not to store all of the router addresses discovered via
advertisements.  However, a host MUST retain at least two router
addresses and SHOULD retain more.  Default router selections are made
whenever communication to a destination appears to be failing.  Thus,
the more routers on the list, the more likely an alternative working
router can be found quickly (e.g., without having to wait for the next
advertisement to arrive).


The algorithm for selecting a router depends in part on whether or not a
router is known to be reachable.  The exact details of how a node keeps
track of a neighbor's reachability state are covered in Section 6.3.
The algorithm for selecting a default router is invoked only when a
Destination Cache entry is incomplete or when communication through an
existing router appears to be failing.  Under normal conditions, a
router would be selected the first time traffic is sent to a
destination, with subsequent traffic for that destination using the same
router as indicated in the Destination Cache.  The policy for selecting
routers from the Default Router List is as follows:

  1) Routers known to be reachable (e.g., in the REACHABLE or PROBE
     state) MUST be preferred over routers whose reachability is unknown
     or suspect.  An implementation may choose to always return the same
     router or cycle through the router list in a round-robin fashion as
     long as it always returns a reachable router when one is available.

  2) When no routers on the list are known to be reachable, routers
     should be selected in a round-robin fashion, so that subsequent



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     requests for a default router do not return the same router until
     all other routers have been selected.

     Cycling through the router list when none are known to be reachable
     ensures that all available routers are actively probed by the
     Neighbor Unreachability Detection algorithm.  A request for a
     default router is made in conjunction with the sending of a packet
     to a router, and the selected router will be probed for
     reachability as a side effect.

  3) If the Default Router List is empty, return an ICMP destination
     unreachable indication with code 0 (no route to destination) to the
     sender of the packet that triggered the search.  Note that if the
     Default Router List contains no entries because none were ever
     added to the Default Router List as a result of received Router
     Advertisements messages, all destinations are assumed to be on-
     link.  Thus, ICMP errors are returned only if the Default Router
     List becomes empty as a result of router Lifetime expirations.




A host is permitted (but not required) to transmit up to
MAX_RTR_SOLICITATIONS Router Solicitation messages from any of its
multicast interfaces after any of the following events:

   - The interface is initialized at system startup time.

   - The interface is reinitialized after a temporary interface failure
     or after being temporarily disabled by system management.

   - The system changes from being a router to being a host, by having
     its IP forwarding capability turned off by system management.

   - The host is re-attached to a link after being detached for some
     time.

The IP destination address of the solicitations is the all-routers
multicast address.  The IP source address MUST be one of the interface's
addresses and MUST be a link-local address.  The Source Link-Layer
Address option is set to the host's link-layer address.

If a host does choose to send a solicitation after one of the above
events, it SHOULD delay that transmission for a random amount of time
between 0 and MAX_RTR_SOLICITATION_DELAY.  This serves to alleviate
congestion when many hosts start up on a link at the same time, such as
might happen after recovery from a power failure.  (It is recommended
that hosts include some unique value, such as one of their IP or link-



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layer addresses, in the seed used to initialize their pseudo-random
number generators.  Although the randomization range is specified in
units of seconds, the actual randomly-chosen value SHOULD not be in
units of whole seconds, but rather in units of the highest available
timer resolution.)

If a host has performed a random delay earlier during the system startup
(e.g.  as part of Duplicate Address Detection [ADDRCONF]) there is no
need to randomly delay the first Router Solicitation message.

A host MAY also choose to further postpone its solicitations, subsequent
to one of the above events, until the first time it needs to use a
default router.

Upon receiving a valid advertisement the host MUST desist from sending
any solicitations on that interface (even if none have been sent yet),
until the next time one of the above events occurs.  The small number of
retransmissions of a solicitation, which are permitted if no such
advertisement is received, SHOULD be sent at intervals of
RTR_SOLICITATION_INTERVAL seconds, without randomization.



6.  ADDRESS RESOLUTION AND NEIGHBOR UNREACHABILITY DETECTION

This section describes the functions related to the Neighbor
Solicitation and Neighbor Advertisement messages and includes
descriptions of the Address Resolution and the Neighbor Unreachability
Detection algorithms.

These messages are also used for Duplicate Address Detection as
specified by [ADDRCONF].  In particular, Duplicate Address Detection
uses the unspecified address as the Source Address in Neighbor
Solicitations to prompt a node already using a particular address to
multicast a Neighbor Advertisement stating that fact.


6.1.  Message Formats


6.1.1.  Neighbor Solicitation Message Format

Nodes send Neighbor Solicitations to request the link-layer address of a
target node while providing their own link-layer address to the target.
Neighbor Solicitations are multicast when the node needs to resolve an
address and unicast when the node seeks to verify the reachability of a
neighbor.




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     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Reserved                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                       Target Address                          +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options ...
     +-+-+-+-+-+-+-+-+-+-+-+-

IP Fields:

   Source Address
                  Either an IP address belonging to the interface from
                  which this message is sent, or the unspecified
                  address.  Use of the unspecified address directs the
                  target node to multicast the resultant Neighbor
                  Advertisement as required by duplicate address
                  detection in [ADDRCONF].

                  A node SHOULD select the same Source Address as the
                  source address in the packet that prompts the
                  solicitation to ensure that the receiver of the
                  solicitation acquires the link-layer address for
                  return traffic.

   Destination Address
                  Either the solicited-node multicast address
                  corresponding to the target address, or the target
                  address.

   Hop Count      1

   Authentication Header
                  If a security association exists between the sender
                  and the destination the sender SHOULD include this
                  header.

ICMP Fields:

   Type           135




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   Code           0

   Checksum       The ICMP checksum.  See [ICMPv6].

   Reserved       This field is unused.  It MUST be initialized to zero
                  by the sender and ignored by the receiver.

   Target Address The IP address of the target of the solicitation.  It
                  MUST NOT be a multicast address.

Options:

   Source link-layer address
                  The link-layer address for the sender.  MUST be
                  included on link layers that have addresses.

   Future versions of this protocol may define new option types.
   Receivers MUST skip over and ignore any options they do not recognize
   and continue processing the message.



6.1.2.  Neighbor Advertisement Message Format

A node MUST send a Neighbor Advertisement in response to a Neighbor
Solicitation for a target IP address that matches an assigned address on
the receiving interface.  A node MAY also send an unsolicited multicast
Neighbor Advertisement when it knows that its link-layer address has
changed.

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |R|S|                       Reserved                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                       Target Address                          +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options ...
     +-+-+-+-+-+-+-+-+-+-+-+-

IP Fields:




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   Source Address
                  An IP address belonging to the interface from which
                  this message is sent.  The source address MUST be the
                  same as the target address for a "normal" (non-
                  anycast/proxy) response.

   Destination Address
                  Either the Source Address of an invoking Neighbor
                  Solicitation, or the all-nodes multicast address.  If
                  the source solicitation is the unspecified address the
                  advertisement MUST be multicast to the all-nodes
                  address.

   Hop Count      1

   Authentication Header
                  If a security association exists between the sender
                  and the destination the sender SHOULD include this
                  header.

ICMP Fields:

   Type           136

   Code           0

   Checksum       The ICMP checksum.  See [ICMPv6].

   R              Router flag.  When set, the R-bit indicates that the
                  sender is a router.  The R-bit is used by Neighbor
                  Unreachability Detection to detect a router that
                  changes to a host.

   S              Solicited flag.  When set, the S-bit indicates that
                  the advertisement was sent in response to a Neighbor
                  Solicitation from the Destination address.  It MUST be
                  zero in a multicast advertisement and in an
                  unsolicited unicast advertisement.

   Reserved       30-bit unused field.  It MUST be initialized to zero
                  by the sender and ignored by the receiver.

   Target Address The address from the Target Address field in the
                  Neighbor Solicitation message that prompted this
                  advertisement.  For an unsolicited advertisement, the
                  address whose link-layer address has changed.  The
                  Target Address MUST NOT be a multicast address.




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Options:

   Target link-layer address
                  The link-layer address for the target.  MUST be
                  included on link layers that have addresses.

   Future versions of this protocol may define new option types.
   Receivers MUST skip over and ignore any options they do not recognize
   and continue processing the message.


6.2.  Address Resolution

Address Resolution provides the mechanism through which nodes determine
the link-layer address of their neighbors.  Address Resolution is only
used for destinations that are determined to be on-link and for which
the sender does not know the corresponding link-layer address.  Address
resolution is never used for multicast destinations.


6.2.1.  Message Validation by Nodes

A node MUST silently discard any received Neighbor Solicitation or
Advertisement messages that do not satisfy all of the following validity
checks:

   - if the message includes an Authentication Header, the message
     authenticates correctly.

   - ICMP Checksum is valid.

   - ICMP Code is 0.

   - ICMP length (derived from the IP length) is 24 or more octets.

   - Target Address is not a multicast address.

   - for a Neighbor Advertisement; if the Destination Address is a
     multicast address the Solicited flag is zero.

   - all included options have a length that is greater then zero.


The contents of the Reserved field, and of any unrecognized options,
MUST be ignored.  Future, backward-compatible changes to the protocol
may specify the contents of the Reserved field or add new options;
backward-incompatible changes may use different Code values.




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Neighbor Solicitations and Advertisements that passes the validity
checks are called "valid solicitations" and "valid advertisements",
respectively.


6.2.2.  Node Specification

When a multicast-capable interface is initialized the node MUST join the
all-nodes multicast address on that interface, as well as the
solicited-node multicast address corresponding to each of the IP
addresses assigned to the interface.

The operation of automatic address configuration [ADDRCONF] may, over
time, change the set of addresses assigned to an interface; new
addresses might be added and old addresses might be removed.  In such
case the node MUST join and leave the solicited-node multicast address
corresponding to the new and old addresses, respectively.  Note that
multiple addresses might correspond to the same solicited-node multicast
address; the host should not leave the multicast address until all
addresses corresponding to the multicast address have been removed.


6.2.3.  Sending Node Specification

When a node has a packet to send, but does not know the next-hop's
link-layer address, it performs address resolution by creating a
Neighbor Cache entry in the INCOMPLETE state and transmitting a Neighbor
Solicitation message targeted at the neighbor.  The packet prompting the
solicitation MUST be queued in the Neighbor Cache entry and the
solicitation MUST be sent to the solicited-node multicast address
corresponding to the target address.

The sender MUST include its link-layer address (if it has one) in the
solicitation as a Source Link-Layer Address option, so that the receiver
discovers the sender's link-layer address without the need for an
additional packet exchange.


While waiting for address resolution to complete, the sender MUST
maintain a small queue containing packets waiting for address resolution
to complete.  The queue MUST hold at least one packet, and MAY contain
more.  However, the number of queued packets per neighbor SHOULD be
limited to some small value.  When a queue overflows, the new arrival
SHOULD replace the oldest entry.  Once address resolution completes, all
queued packets SHOULD be transmitted.

While awaiting a response, the sender MUST retransmit Neighbor
Solicitation messages approximately every RESOLVE_RETRANS_TIMER seconds,



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even in the absence of additional traffic to the neighbor.
Retransmissions MUST be rate-limited for each neighbor to at most one
solicitation every RESOLVE_RETRANS_TIMER seconds.

If no advertisement is received after MAX_MULTICAST_SOLICIT
solicitations, address resolution has failed.  The sender MUST return
ICMP destination unreachable indications with code 3 (Address
Unreachable) for each packet queued awaiting address resolution.

When a valid Neighbor Advertisement is received (either solicited or
unsolicited), the Neighbor Cache is searched for the target's entry.  If
no entry exists, the advertisement SHOULD be silently discarded.  There
is no need to create an entry, since the recipient has apparently not
initiated any communication with the target.

If an INCOMPLETE Neighbor Cache entry exists for the target, the
advertisement is the first response to a solicitation.  In such cases,
the receiving node records the link-layer address in the Neighbor Cache
entry and sends any packets queued for the neighbor awaiting address
resolution.  In addition, the receiving node MUST examine the Router
flag field of the advertisement and update the "is_router" flag in the
Neighbor Cache entry to reflect whether the node is a host or router.
If the neighbor was previously behaving as a router, but the
advertisement's Router flag is set to zero, the node MUST update the
Neighbor Cache entries for all destination using that neighbor as a
reouter as indicated in Section 6.3.2.

If the Solicited flag is set, the node MUST update the reachability
information as described in Section 6.3.2 by setting the state to
REACHABLE.  In addition, the receiving node MUST examine the Router flag
field of the advertisement and update the "is_router" flag in the
Neighbor Cache entry to reflect whether the node is a host or router.
If the neighbor was previously behaving as a router, but the
advertisement's Router flag is set to zero, the node MUST update the
Neighbor Cache entries for all destination using that neighbor as a
reouter as indicated in Section 6.3.2.

Multiple solicited Neighbor Advertisements can be received in response
to a solicitation for a anycast/proxy address.  In such cases one or
more of the advertisements is a anycast/proxy advertisement.
Anycast/proxy advertisements are identified by having differing source
and target addresses.  A node MUST give preference to "normal" responses
over anycast/proxy responses and, among multiple anycast/proxy
responses, a node MUST prefer the first anycast/proxy response.  This is
accomplished by applying the following rules while processing received
advertisements with the Solicited flag set to one:

   1) if no link-layer address is currently recorded, install the one



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     contained in the advertisement.

   2) if a link-layer address has already been recorded, and the
     advertisement is not an anycast/proxy advertisement, replace the
     recorded address with the one contained in the advertisement.

   3) otherwise ignore the advertisement


A node MAY occasionally multicast or unicast an unsolicited Neighbor
Advertisement announcing a link-layer address change.  A node that
receives a Neighbor Advertisement without the Solicited flag set, does
the following:

   - If the node has a Neighbor Cache entry for the target, it SHOULD
     copy the link-layer address information contained in the
     advertisement's Source Link-Layer Address option into the
     corresponding Neighbor Cache entry.

   - The node MUST not treat the receipt of a unsolicited advertisement
     as a confirmation that the neighbor is REACHABLE (as defined in
     Section 4.1).  See Section 6.3.1.


6.2.4.  Target Node Specification

When a node receives a valid Neighbor Solicitation, it compares the
message's Target Address against the IP addresses belonging to the
incoming interface as well as any anycast addresses that it has been
configured to respond to.  If no match is found, the node is not the
target of the solicitation and it MUST silently ignore the message.

If the node is the target of the solicitation, and the solicitation's
Source Address is not the unspecified address, the recipient first
ensures that it has an up-to-date Neighbor Cache entry for the Source
Address of the solicitation.  If no entry is found one is created and
its link-layer address is copied from the Source Link-Layer Address
option in the message.  If an entry already exists, its link-layer
address is updated to match the address in the Source Link-Layer Address
option.  If a Neighbor Cache entry is created for the source its
reachability state MUST be set to PROBE as specified in Section 6.3.2.

If the source of the solicitation is the unspecified address, the target
MUST multicast an advertisement to the all-nodes address.  Otherwise,
the target MUST send a unicast Neighbor Advertisement to the address
copied from the IP Source Address of the Neighbor Solicitation.  In both
cases the Target Address is copied from the solicitation message to the
advertisement and the Target Link-Layer Address option is filled with



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the node's link-layer address on the link.  If the advertisement is
multicast the Solicited flag MUST be set to zero; if it is unicast the
Solicited flag MUST be set to one in order to give the source a
reachability confirmation.  If the node is a router it MUST set the
Router flag to one; otherwise it MUST be set to zero.

If the node is not providing anycast/proxy services for the targeted
address, the IP Source Address MUST be set to the address in the Target
Address field (which is one of the IP addresses belonging to the
interface).  Doing so guarantees that the receiver can identify the
Neighbor Advertisement as being a "normal" advertisement.

If the node is providing anycast/proxy services for the target the IP
Source Address MUST be set the interface's designated address (which is
different than the Target Address) so that the receiver recognizes the
message as an anycast/proxy advertisement.


6.2.5.  Anticipated Link-Layer Address Changes

In some cases a node may be able to determine that its link-layer
address has changed (e.g., hot-swap of an interface card) and may wish
to inform its neighbors of the new link-layer address quickly.  In such
cases a node MAY send up to MAX_NEIGHBOR_ADVERTISEMENT unsolicited
Neighbor Advertisement messages to the all-nodes multicast address.
These advertisements MUST be separated by at least
MIN_NEIGHBOR_ADVERT_INTERVAL seconds.

The Target Address field in the unsolicited advertisement is set to an
IP address of the interface and the Target Link-Layer Address option is
filled with the new link-layer address.  The Solicited flag MUST be set
to zero, in order to avoid confusing the Neighbor Unreachability
Detection algorithm.  If the node is a router is MUST set the Router
flag to one; otherwise it MUST be set to zero.

A node that has multiple IP addresses assigned to an interface MAY
multicast a separate Neighbor Advertisement for each address.  In such a
case the node SHOULD introduce a small delay between the sending of each
advertisement to reduce the probability of the advertisements being
dropped by hosts.

A proxy MAY multicast Neighbor Advertisements when its link-layer
address changes or when it is configured (by system management or other
mechanisms) to proxy for an address.  If there are multiple nodes that
are providing proxy services for the same set of addresses the proxies
SHOULD provide a mechanism that prevents multiple proxies from
multicasting advertisements for the same addresses.




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Note that unsolicited Neighbor Advertisements do not reliably update
caches in all nodes (the advertisements might not be received by all
nodes) and should only be viewed as a performance optimization to
quickly update the caches in most neighbors.  The Neighbor
Unreachability Detection algorithm will ensure that neighbors reliably
update the cached link-layer address when they attempt to communicate
with the node.


6.2.6.  Anycast Neighbor Advertisements

An anycast address cannot be syntactically distinguished from other
unicast addresses.  This section shows how the rules defined above "do
the right thing" for anycast addresses.

A node belonging to an anycast address MUST join the solicited-node
multicast address that corresponds to the anycast address.

When a node responds to a Neighbor Solicitation for an anycast address,
it by definition responds with a anycast/proxy Neighbor Advertisement.
Anycast address are not permitted to appear as the source address in an
IP packet, guaranteeing that the advertisement's source and target
addresses differ.

A node might receive multiple Neighbor Advertisements in response to a
Neighbor Solicitation for an anycast address when multiple neighbors are
configured to recognize the anycast address.  The precedence rules in
Section 6.2.3 will make the node select the first advertisement (i.e.
the fastest or most lightly loaded node) as current binding for the
anycast address.

The use of Neighbor Unreachability Detection ensures that a node quickly
detects when the current binding for the anycast address has gone stale
e.g. due to a node no longer belonging to the anycast address.


6.2.7.  Proxy Neighbor Advertisements

Under limited circumstances, a router MAY proxy for one or more other
nodes, that is, through Neighbor Advertisements indicate that it is
willing to accept packets not explicitly addressed to itself.  For
example, a router might potentially accept packets on behalf of a mobile
node that has moved off-link.  The mechanisms used by proxy are
identical to the mechanisms needed for anycast addresses.  The address
being served is called a "proxee" in this section.

A proxy MUST join the solicited-node multicast address(es) that
correspond to the proxee's IP address(es).



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All proxy Neighbor Advertisement messages MUST be tagged as being
anycast/proxy messages; the advertisement's Source Address MUST differ
from its Target Address (e.g., the proxee).  In practice, this
requirement poses no special burden.  By definition, the advertisement's
source address MUST be the designated address of the interface on which
the advertisement is sent, which will be different than any proxee
address.


6.3.  Neighbor Unreachability Detection

Communication to or through a neighbor may fail for numerous reasons at
any time, including hardware failure, hot-swap of an interface card, a
mobile node moving off-link, etc.  If the destination has failed, no
recovery is possible and communication fails.  On the other hand, if it
is the path that has failed, recovery may be possible.  Thus, a node
actively tracks the reachability "state" for the neighbors to which it
is sending packets.

Neighbor Unreachability Detection is used for all paths between hosts
and neighboring nodes, including host-to-host, host-to-router, and
router-to-host communication.  Neighbor Unreachability Detection may
also be used between routers, but is not required if an equivalent
mechanism is available, for example, as part of the routing protocols.

When a path to a neighbor appears to be failing, the specific recovery
attempt depends on how the neighbor is being used.  For example,
appropriate recovery procedures when using the neighbor as a router
differ from those appropriate for the case where the neighbor is the
destination.

Neighbor Unreachability Detection is performed only for neighbors to
which unicast packets are sent; it is not used when sending to multicast
addresses.


6.3.1.  Reachability Confirmation

A neighbor is considered reachable if the node has recently received a
confirmation that packets sent to the neighbor are received by its IP
layer.  Positive confirmation can be gathered in two ways: hints from
upper layer protocols that indicate a connection is making "forward
progress", or receipt of a Neighbor Advertisement message that is a
response to an explicit Neighbor Solicitation probe.

A connection makes "forward progress" if the packets received from a
remote peer can only be arriving if recent packets sent to that peer are
actually reaching it.  For example, receipt of a (new) acknowledgement



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indicates that previously sent data reached the peer.  Likewise, the
arrival of a new (non-duplicate) packet indicates that earlier
acknowledgements are being delivered to the remote peer.  If packets are
reaching the peer the packets must also be reaching the sender's next-
hop neighbor; thus "forward progress" is a confirmation that the next-
hop neighbor is reachable.  When available, this upper-layer information
SHOULD be used.

In some cases (e.g, UDP-based protocols and routers forwarding packets
to hosts) such reachability information may not be readily available
from upper-layer protocols.  When no hints are available and a node is
sending packets to a neighbor, the node actively probes the neighbor
using Neighbor Solicitation messages to verify that the forward path is
still working.

The receipt of a solicited Neighbor Advertisement that is a response to
a Neighbor Solicitation probe serves as reachability confirmation, since
advertisements with the Solicited flag set to one are sent only in
response to a solicitation.  A received Neighbor Advertisement with the
Solicited flag set to zero MUST NOT be treated as a reachability
confirmation.  Receipt of unsolicited advertisements only confirm the
one-way path from the neighbor to the recipient node.  In contrast,
Neighbor Unreachability Detection requires that a path be working from
the sender to the neighbor.  An advertisement sent in response to an
explicit solicitation confirms that a path is working in both
directions; the solicitation reached the neighbor, prompting it to
generate an advertisement, and the advertisement reached the querying
node.


6.3.2.  Node Behavior

Neighbor Unreachability Detection operates in parallel with the sending
of packets to a neighbor.  While reasserting a neighbor's reachability,
a node continues sending packets to that neighbor using the cached
link-layer address.

A Neighbor Cache entry can be in one of three states:

   INCOMPLETE  Address resolution is being performed on the entry.
               Specifically, a Neighbor Solicitation has been sent to
               the solicited-node multicast address of the target, but
               the corresponding Neighbor Advertisement has not yet been
               received.

   REACHABLE   Positive confirmation was received within the last
               ReachableTime milliseconds that the forward path to the
               neighbor was functioning properly.  While REACHABLE, no



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               special action takes place as packets are sent.

   PROBE       More than ReachableTime milliseconds have elapsed since
               the last positive confirmation was received that the
               forward path was functioning properly.  Upon entering the
               PROBE state, the sending of an initial Neighbor
               Solicitation is delayed by a time of
               DELAY_FIRST_PROBE_TIME to give the upper layers
               additional time to provide reachability confirmation
               information.  After the initial delay, Neighbor
               Solicitations are sent every ReachableRetransTimer
               milliseconds.

When an entry is created as a result of needing to perform address
resolution, a Neighbor Solicitation is sent to the solicited-node
multicast address of the target, a timer is started to expire
RESOLVE_RETRANS_TIMER seconds later and the entry's state is set to
INCOMPLETE.

As specified in Section 6.2.3, when in the INCOMPLETE state, Neighbor
Solicitation messages are retransmitted every RESOLVE_RETRANS_TIMER
seconds until a response is received.  If no response is received within
RESOLVE_RETRANS_TIMER seconds after sending MAX_MULTICAST_SOLICIT probes
to the solicited-node multicast address, address resolution fails.  Upon
failure, ICMP destination unreachable indications with code 3 (Address
unreachable) are returned for any queued packets and the entry is
deleted.  Note that deleting the entry implies that all destinations
using that neighbor must perform next-hop resolution again before
sending a subsequent packet.  Thus, if the neighbor is a router, an
alternate router may be selected.  Alternatively, a destination
previously thought to be on-link, may now only be reachable through a
router.

Unreachability detection changes a neighbor's state from REACHABLE to
PROBE only on-demand, when a packet is being sent to that neighbor.
When no traffic is sent to a neighbor, an entry may technically no
longer be in a REACHABLE state, but the condition is not checked or
acted upon until a packet is sent to the neighbor.

The first time a Neighbor Cache entry is referenced and more than
ReachableTime milliseconds have passed since the last reachability
confirmation was received, its state changes to PROBE.  However, no
Neighbor Solicitation probe is sent.  Probing is deferred for an
additional DELAY_FIRST_PROBE_TIME seconds, an optimization that gives
the upper-layer protocol additional time to provide a reachability
confirmation in those cases where ReachableTime milliseconds have passed
since the last confirmation due to lack of recent traffic.  Without this
optimization the opening of a TCP connection after a traffic lull would



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initiate probes even though the subsequent three-way handshake would
provide a reachability confirmation almost immediately.

If no reachability confirmation is received within
DELAY_FIRST_PROBE_TIME seconds after entering the PROBE state, a unicast
Neighbor Solicitation message is sent to the neighbor using the cached
link-layer address.  In addition, the sender starts a timer to
retransmit probe messages every ReachableRetransTimer milliseconds until
the desired solicitation is received.  Subsequent probes are
retransmitted even if no additional packets are sent to the neighbor.
If no response is received after waiting ReachableRetransTimer
milliseconds after sending the MAX_UNICAST_SOLICIT solicitations,
retransmissions cease and the entry SHOULD be deleted.  Subsequent
traffic to that neighbor recreates the entry and performs address
resolution again.

Note that all Neighbor Solicitations are rate-limited on a per-neighbor
basis.  A node MUST NOT send Neighbor Solicitations to the same neighbor
more frequently than once every ReachableRetransTimer milliseconds.

The PROBE state is also used when creating Neighbor Cache entries as a
result of receiving packets other than solicited Neighbor
Advertisements.  This includes Router Solicitations, Router
Advertisements, Redirects, and Neighbor Solicitations.  These packets
contain the link-layer of either the sender or, in the case of Redirect,
the redirection target.  However, receipt of the link-layer address does
not confirm reachability of the forward-direction path to the node.
Placing a newly created Neighbor Cache entry for which the link-layer
address is known in the PROBE state provides assurance that path
failures are detected quickly.

To detect a router that switches from being a router to being a host
(e.g, by having its IP forwarding capability turned off by system
management), a node MUST compare the Router flag field in all received
Neighbor Advertisement messages with the "is_router" flag recorded in
the Neighbor Cache entry.  When a node detects that a neighbor has
changed from being a router to being a host, the node MUST remove that
router from the Default Router List and update the Destination Cache so
that all entries using that neighbor as a router switch to another
router.  Note that a router may not be listed in the Default Router
List, but still have Destination Cache entries using it, if a host was
redirected to it.









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7.  REDIRECT FUNCTION

This section describes the set of functionality related to the sending
and processing of Redirect messages.


7.1.  Redirect Message Format

A Redirect packet is sent from a router to a host to inform the host of
a better first-hop node on the path to a destination.

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |     Code      |          Checksum             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Reserved                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                       Target Address                          +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                     Destination Address                       +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options ...
     +-+-+-+-+-+-+-+-+-+-+-+-

IP Fields:

   Source Address
                  The designated address of the interface from which the
                  redirect is sent.

   Destination Address
                  The Source Address of the packet that triggered the
                  redirect.

   Hop Count      1

   Authentication Header



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                  If a security association exists between the sender
                  and the destination the sender SHOULD include this
                  header.

ICMP Fields:

   Type           5

   Code           0

   Checksum       The ICMP checksum.  See [ICMPv6].

   Reserved       This field is unused.  It MUST be initialized to zero
                  by the sender and ignored by the receiver.

   Target Address An IP address of the node to which traffic for the
                  Destination SHOULD be sent.  When the target is a
                  router, the Target Address MUST be the router's
                  designated address so that hosts can uniquely identify
                  routers.  When the target is a host the target address
                  field MUST be set to the same value as the Destination
                  Address field.

   Destination Address
                  The IP address of the destination which is redirected
                  to the target.

Options:

   Target link-layer address
                  The link-layer address for the target.  It SHOULD be
                  included on link layers that have addresses, if known.

   Redirected Header
                  As much as possible of the IP packet that triggered
                  the sending of the Redirect without making the
                  redirect packet exceed 576 octets.

   Future versions of this protocol may define new option types.
   Receivers MUST skip over and ignore any options they do not recognize
   and continue processing the message.


7.2.  Router Specification

A router SHOULD send a redirect message, subject to rate limiting,
whenever it forwards a packet in which:




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   - the Source Address field of the packet identifies a neighbor, and

   - after consulting its routing table, the router forwards the packet
     to a node residing on the same link as the packet's source, and

   - the Destination Address of the packet is not a multicast address,
     and

   - the packet is not source routed through the router.  A packet is
     source routed through the router if, when the packet is received by
     the router, it contains the IP route header and the router's
     address is in the Destination Address field.


The transmitted redirect packet contains, consistent with the above
message format:

   - In the Target Address field: the address to which subsequent
     packets for the destination SHOULD be sent.  If the target is a
     router, that router's designated address MUST be used.  If the
     target is a host the target address field MUST be set to the same
     value as the Destination Address field.

   - In the Destination Address field: the destination address of the
     invoking IP packet.

   - In the options:

     Target Link-Layer Address option: link-layer address of the target,
          if known.

     Redirected Header: as much of the forwarded packet as can fit
          without the redirect packet exceeding 576 octets in size.

A router MUST limit the rate of Redirect messages sent, in order to
limit the bandwidth and processing costs incurred by the Redirect
messages when the source does not correctly respond to the Redirects, or
the source chooses to ignore unauthenticated Redirect messages.  More
details on the rate-limiting of ICMP error messages can be found in
[ICMPv6].

A router MUST NOT update its routing tables upon receipt of a Redirect.


7.3.  Host Specification






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7.3.1.  Message Validation by Hosts

A host MUST silently discard any received Redirect messages that do not
satisfy all of the following validity checks:


   - IP Source Address is a link-local address.

   - ICMP Checksum is valid.

   - ICMP Code is 0.

   - ICMP length (derived from the IP length) is 40 or more octets.

   - the IP source address of the Redirect is the same as the current
     first-hop router for the specified destination.

   - the Target Address of the redirect is not a multicast address.

   - the Destination Address field in the redirect message does not
     contain a multicast address.

   - if the message includes an Authentication Header, the message
     authenticates correctly.

   - all included options have a length that is greater then zero.


The contents of the Reserved field, and of any unrecognized options MUST
be ignored.  Future, backward-compatible changes to the protocol may
specify the contents of the Reserved field or add new options;
backward-incompatible changes may use different Code values.

A host MUST NOT consider a redirect invalid just because the Target
Address of the redirect is not covered under one of the link's prefixes.

A redirect that passes the validity checks is called a "valid redirect".


7.3.2.  Host Behavior

A host receiving a valid redirect SHOULD update its routing information
accordingly.  When a redirect is received, the host updates the
Destination Cache entry for the destination to use to the specified
target as the new next-hop.  If no Destination Cache entry exists for
the destination such an entry is created (placing it in the PROBE
state).




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If the redirect contains a Target Link-Layer Address option the host
either creates or updates the Neighbor Cache entry for the target.  The
link-layer address in the Neighbor Cache entry MUST be copied from the
Target Link-Layer Address option.  If a Neighbor Cache entry is created
for the target its reachability state MUST be set to PROBE as specified
in Section 6.3.2.  In addition, if the Target Address is the same as the
Destination Address the host as been redirected to the destination and
it should set the "is_router" field in a created Neighbor Cache entry to
false.  Otherwise it should set to true.

A host MAY have a configuration switch that can be set to make it ignore
a Redirect message that does not have an IP Authentication header.


A host MUST NOT send Redirect messages.


8.  OPTIONS

Options provide a mechanism for encoding variable length fields, fields
that may appear multiple times in the same packet, or information that
is optional and may not appear in all packets.  Options can also be used
to add additional functionality to future versions of ND.

In order to ensure that future extensions can properly coexist with
current implementations, all nodes MUST skip over any options they do
not recognize in received ND packets and continue processing the packet.
However, the options specified in this document MUST be implemented by
all implementations.

The current set of options is defined in such a way that receivers can
process multiple options in the same packet independently of each other.
In order to maintain these properties future options SHOULD follow the
simple rule:

     The option MUST NOT depend on the presence or absence of any other
     options.  The semantics of an option should depend only on the
     information in the fixed part of the ND packet and on the
     information contained in the option itself.

Adhering to the above rule has the following benefits:

  1) Receivers can process options independently of one another.  For
     example, an implementation can choose to process the Prefix
     Information option in a Router Advertisement message in a user-
     space process while the link-layer address in the same message is
     processed by routines in the kernel.




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  2) Should the number of options cause a packet to exceed a link's MTU,
     multiple packets can carry subsets of the options without any
     change in semantics.

  3) Senders MAY send a subset of options in different packets.  For
     instance, if the prefix Invalidation Lifetime is high it might not
     be necessary to include the Prefix Information option in every
     Router Advertisement.  In addition, different routers might send
     different sets of options.  Thus, a receiver MUST NOT associate any
     action with the absence of an option in a particular packet.  This
     protocol specifies that receivers should only act on the expiration
     of timers and on the information that is received in the packets.



When multiple options are present in a Neighbor Discovery packet, they
may appear in any order; receivers MUST be prepared to process them
independently of their order.  There can also be multiple instances of
the same option in a message, for instance Prefix Information options.

All options have a length that is a multiple of 8 octets, ensuring
appropriate alignment without any "pad" options.  The fields in the
options, as well as the fields in ND packets, are defined to align on
their natural boundaries (e.g. a 16-bit field is aligned on a 16-bit
boundary) except the 128-bit IP addresses/prefixes which are aligned on
a 64-bit boundary.

The link-layer address field contains an octet string; thus it is only
aligned on an 8-bit boundary.

All options are of the form:

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |    Length     |              ...              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~                              ...                              ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:

   Type           8-bit identifier of the type of option.  The options
                  defined in this document are:









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                        Option Name                             Type

                     Source Link-Layer Address                    1
                     Target Link-Layer Address                    2
                     Prefix Information                           3
                     Redirected Header                            4
                     MTU                                          5

   Length         8-bit unsigned integer.  The length of the option in
                  units of 8 octets.  The value 0 is invalid.  Nodes
                  MUST silently discard an ND packet that contains an
                  option with length zero.

The size of an ND packet including the IP header is limited to the link
MTU (which is at least 576 octets).  When adding options to an ND packet
a node MUST NOT exceed the link MTU.  This is handled in a packet
specific manner.

The only ND packets that currently can exceed the link MTU are Router
Advertisements and Redirects; the former due a large number of Prefix
Information options and the latter due to the Redirected Header option.

If there are more Prefix Information options than can fit in a single
Router Advertisement packet the router MUST send multiple separate
advertisements that each contain a subset of the set of prefixes.

In a Redirect packet the amount of data included in the Redirected
Header MUST be limited so that the packet does not exceed 576 octets.



8.1.  Source/Target Link-layer Address

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |    Length     |    Link-Layer Address ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:

   Type
                  1 for Source Link-layer Address
                  2 for Target Link-layer Address

   Length         The length of the option in units of 8 octets.  For
                  example, the length with IEEE 802 addresses is 1.

   Link-Layer Address
                  The variable length link-layer address.



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                  The content and format of this field is expected to be
                  specified in specific documents that describe how IPv6
                  operates over different link layers.  The format for
                  IEEE 802 addresses is a 6-byte string that represents
                  the address in Canonical bit order.

Description
                  The Source Link-Layer address option contains the
                  link-layer address of the sender of the packet.  It is
                  used in the Neighbor Solicitation, Router
                  Solicitation, and Router Advertisement packets.

                  The Target Link-Layer address option contains the
                  link-layer address of the target.  It is used in
                  Neighbor Advertisement and Redirect packets.


8.2.  Prefix Information

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |    Length     | Prefix Length |L|A| Reserved1 |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Invalidation Lifetime                    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                      Deprecation Lifetime                     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Reserved2                           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     +                                                               +
     |                                                               |
     +                            Prefix                             +
     |                                                               |
     +                                                               +
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:

   Type           3

   Length         4

   Prefix Length  8-bit unsigned integer.  The number of leading bits in
                  the Prefix that are valid.  The value ranges from 0 to
                  128.

   L              1-bit on-link flag.  When set, indicates that this



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                  prefix can be used for on-link determination.

   A              1-bit autonomous address-configuration flag.  When set
                  indicates that this prefix can used for autonomous
                  address configuration as specified in [ADDRCONF].

   Reserved1      6-bit unused field.  It MUST be initialized to zero by
                  the sender and ignored by the receiver.

   Invalidation Lifetime
                  32-bit unsigned integer.  The lifetime of the prefix
                  in seconds for the purpose of on-link determination.
                  A value of all one bits (2^32-1) represents infinity.
                  This lifetime is also used by [ADDRCONF].

   Deprecation Lifetime
                  32 bits reserved for autonomous address configuration.
                  A value of all one bits (2^32-1) represents infinity.
                  See [ADDRCONF].

   Reserved2      This field is unused.  It MUST be initialized to zero
                  by the sender and ignored by the receiver.

   Prefix         An IP address or a prefix of an IP address.  The
                  prefix length field contains the number of valid
                  leading bits in the prefix.

Description
                  The Prefix Information option is only used in Router
                  Advertisement packets.  It provide hosts with on-link
                  prefixes and prefixes for Address Autoconfiguration.

                  Implementations can choose to process the on-link
                  aspects of the prefixes separately from the address
                  autoconfiguration aspects of the prefixes by e.g.
                  passing a copy of each valid Router Advertisement
                  message to both an "on-link" and an "addrconf"
                  function.  Each function can then operate on the
                  prefixes that have the appropriate flag set.


8.3.  Redirected Header









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     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |    Length     |            Reserved           |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Reserved                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                       IP header + data                        ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:

   Type           4

   Length         The length of the option in units of 8 octets.

   Reserved       These fields are unused.  They MUST be initialized to
                  zero by the sender and ignored by the receiver.

   IP header + data
                  The original packet truncated to ensure that the size
                  of the redirect message does not exceed 576 octets.

Description
                  The Redirected Header option MUST be included in
                  Redirect packets.


8.4.  MTU

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Type      |    Length     |           Reserved            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              MTU                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:

   Type           5

   Length         1

   Reserved       This field is unused.  It MUST be initialized to zero
                  by the sender and ignored by the receiver.

   MTU            32-bit unsigned integer.  The recommended MTU for the
                  link.




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Description
                  The MTU option SHOULD be included in Router
                  Advertisement packets when the link has no well-known
                  MTU and it MAY be included on links with a well-known
                  MTU.

                  Hosts MUST handle this option by setting the LinkMTU
                  variable for the interface to the received value.  If
                  the routers on the link are advertising different MTU
                  values this will result in hosts switching between the
                  different MTUs.  Therefore, routers SHOULD verify the
                  consistency between the MTU they advertise and that
                  other routers advertise and log a network management
                  event when there is a mismatch.

                  When a host or its interface is initialized the
                  LinkMTU of the interface should be set to the
                  predefined value for that type of link.  If the host
                  receives no MTU option it must continue to use that
                  predefined value.  The MTU option can be used by
                  routers to both increase and decrease the MTU.




9.  MULTIHOMED HOSTS

There are a number of complicating issues that arise when Neighbor
Discovery is used by hosts that have multiple interfaces.  This section
does not attempt to define the proper operation of multihomed hosts with
regard to Neighbor Discovery.  Rather, it identifies complicating issues
that require further study.  Implementors are encouraged to experiment
with various approaches to making Neighbor Discovery work on multihomed
hosts and to report their experiences.

If a multihomed host receives Router Advertisements on all of its
interfaces, it will (probably) have learned on-link prefixes for the
addresses residing on each link.  When a packet must be sent through a
router, however, selecting the "wrong" router can result in a suboptimal
or non-functioning path.  There are two cases to consider:

  1) If the first-hop router knows of a better first-hop on the same
     link as the router and sending host, it can send a redirect,
     provided that the source address of the offending packet matches an
     address on the outgoing interface (e.g., the source address resides
     on the same subnet as the router).  On non-multihomed hosts, this
     will always be the case.  On multihomed hosts, however, the
     packet's source address may be associated with another interface.



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     In such cases, no redirect will be sent and suboptimal routing
     cannot be corrected.

  2) If the selected first-hop router does not have a route at all for
     the destination, it will be unable to deliver the packet.  However,
     the destination may be reachable through a router on one of the
     other interfaces.  Neighbor Discovery does not address this
     scenario; it does not arise in the non-multihomed case.

  3) Even if the first-hop router does have a route for a destination,
     there may be a better route via another interface.  No mechanism
     exists for the multihomed host to detect this situation.

If a multihomed host fails to receive Router Advertisements on one or
more of its interfaces, it will not know (in the absence of configured
information) which destinations are on-link on the affected
interface(s).  This leads to a number of problems:

  1) If no Router Advertisement is received on any interfaces, a
     multihomed host will have no way of knowing which interface to send
     packets out on, even for on-link destinations.  Under similar
     conditions in the non-multihomed host case, a node treats all
     destinations as residing on-link, and communication proceeds.  In
     the multihomed case, however, additional information is needed to
     select the proper outgoing interface.  Alternatively, a node could
     attempt to perform address resolution on all interfaces.  However,
     this entails significant complexity that is not present in the
     non-multihomed host case.

  2) If Router Advertisements are received on some, but not all
     interfaces, a multihomed host could choose to only send packets out
     on the interfaces on which it has received Router Advertisements.
     A key assumption made here, however, is that routers on those other
     interfaces will be able to route packets to the ultimate
     destination, even when those destinations reside on the subnet to
     which the sender connects, but has no on-link prefix information.
     Should the assumption be false, communication would fail.  Even if
     the assumption holds, packets will traverse a sub-optimal path.













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10.  PROTOCOL CONSTANTS

Router constants:
         MAX_INITIAL_RTR_ADVERT_INTERVAL  16 seconds

         MAX_INITIAL_RTR_ADVERTISEMENTS    3 transmissions

         MAX_RTR_RESPONSE_DELAY            2 seconds

Host constants:

         MAX_RTR_SOLICITATION_DELAY        1 second

         RTR_SOLICITATION_INTERVAL         3 seconds

         MAX_RTR_SOLICITATIONS             3 transmissions

Node constants:

         RESOLVE_RETRANS_TIMER             1 second

         MAX_MULTICAST_SOLICIT             3 transmissions

         MAX_UNICAST_SOLICIT               3 transmissions

         MAX_NEIGHBOR_ADVERTISEMENT        3 transmissions

         MIN_NEIGHBOR_ADVERT_INTERVAL     16 seconds

         REACHABLE_TIME               30,000 milliseconds

         REACHABLE_RETRANS_TIMER       3,000 milliseconds

         DELAY_FIRST_PROBE_TIME            3 seconds

Additional protocol constants are defined with the message formats in
Section 5.1, 6.1, and 7.1.

All protocol constants are subject to change in future revisions of the
protocol.


11.  SECURITY CONSIDERATIONS

Neighbor Discovery protocol packet exchanges can be authenticated using
the IP Authentication Header [IPv6-AUTH].  A node SHOULD include an
Authentication Header when sending Neighbor Discovery packets if a
security association exists for the destination address.  The security



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associations may have been created through manual configuration or
through the operation of some key management protocol.

Received Authentication Headers in Neighbor Discovery packets MUST be
verified for correctness and packets with incorrect authentication MUST
be ignored.

It SHOULD be possible for the system administrator to configure a node
to ignore any Neighbor Discovery messages that are not authenticated
using either the Authentication Header or Encapsulating Security
Payload.  The configuration technique for this MUST be documented.  Such
a switch SHOULD default to allowing unauthenticated messages.

Confidentiality issues are addressed by the IP Security Architecture and
the IP Encapsulating Security Payload documents [IPv6-SA, IPv6-ESP].

The trust model for redirects is based only on trusting a redirect
received from the current first-hop node as in IPv4.  It is natural to
trust the routers on the link.  If a host has been redirected to another
host (i.e. the destination is on-link) there is no way to prevent the
target from issuing another redirect to some other destination.
However, this exposure is no worse than it was; the target host, once
subverted, could always act as a hidden router to forward traffic
elsewhere.

The protocol contains no mechanism to determine which nodes are
authorized to send Router Advertisements; any node, presumably even in
the presence of authentication, can send Router Advertisement messages
thereby being able to cause denial of service.  Furthermore, any node
can send proxy Neighbor Advertisements as well as unsolicited Neighbor
Advertisements as a potential denial of service attack.




















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REFERENCES


  [ADDRCONF] S. Thomson, "IPv6 Address Autoconfiguration", Internet
          Draft.

  [ADDR-ARCH] S. Deering, R. Hinden, Editors, "IP Version 6 Addressing
          Architecture", Internet Draft.

  [ANYCST] C. Partridge, T. Mendez, and W. Milliken, "Host Anycasting
          Service", RFC 1546, November 1993.

  [ARP] D. Plummer, "An Ethernet Address Resolution Protocol", STD 37,
          RFC 826, November 1982.

  [HR-CL] R. Braden, Editor, "Requirements for Internet Hosts --
          Communication Layers", STD 3, RFC 1122, October 1989.

  [ICMPv4] J. Postel, "Internet Control Message Protocol", STD 5, RFC
          792, September 1981.

  [ICMPv6] A. Conta, and S. Deering, "ICMP for the Internet Protocol
          Version 6 (IPv6)", Internet Draft.

  [IPv6] S. Deering, R. Hinden, Editors, "Internet Protocol, Version 6
          (IPv6) Specification", Internet Draft.

  [IPv6-SA] R. Atkinson.  IPv6 Security Architecture.  Internet Draft,
          March 1995.

  [IPv6-AUTH] R. Atkinson.  IPv6 Authentication Header.  Internet Draft,
          March 1995.

  [IPv6-ESP] R. Atkinson.  IPv6 Encapsulating Security Payload.
          Internet Draft, February 1995.

  [RDISC] S. Deering, "ICMP Router Discovery Messages", RFC 1256,
          September 1991.

  [SH-MEDIA] R. Braden, J. Postel, Y. Rekhter, "Internet Architecture
          Extensions for Shared Media", RFC 1620, May 1994.

  [ASSIGNED] J. Reynolds, J. Postel, "ASSIGNED NUMBERS", RFC 1700,
          October 1994.

  [SYNC] S. Floyd, V. Jacobsen, "The Synchronization of Periodic Routing
          Messages", IEEE/ACM Transactions on Networking, April 1994.
          ftp://ftp.ee.lbl.gov/papers/sync_94.ps.Z



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AUTHORS' ADDRESSES

     Erik Nordmark                Thomas Narten
     Sun Microsystems, Inc.       IBM Corporation
     2550 Garcia Ave              P.O. Box 12195
     Mt. View, CA 94041           Research Triangle Park, NC 27709-2195
     USA                          USA

     phone: +1 415 336 2788       phone: +1 919 254 7798
     fax:   +1 415 336 6015       fax:   +1 919 254 4027
     email: nordmark@sun.com      email: narten@vnet.ibm.com

     William Allen Simpson
     Daydreamer
     Computer Systems Consulting Services
     1384 Fontaine
     Madison Heights, Michigan  48071
     USA

     email: Bill.Simpson@um.cc.umich.edu
            bsimpson@MorningStar.com






























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CHANGES SINCE PREVIOUS DOCUMENT

     There are several changes since the previous version documented in:
          <draft-ietf-ipngwg-discovery-00.txt>
     based on feedback from the working group:

          o Renamed the "Next-hop Cache" to the "Destination Cache".

          o Renamed "extensions" to "options".

          o Renamed "lifetime-as-default" to "Router Lifetime".

          o Clarified that on-link is a property of an address whereas
            neighbor is a property of a node.

          o Changed the solicited-node multicast address range from 256
            addresses to 4 billion (2^32) addresses.

          o Removed use of all-hosts multicast address.  Router
            Advertisement messages are now sent to all-nodes.  This
            allows routers to verify the consistency of the information
            different routers advertise.

          o Removed the preference field from Router Advertisements.
            Simplified the default router selection algorithm as a
            result.

          o Moved the information carried in the MTU and NUD timer
            extensions into the fixed part of the RA header.  Added an
            additional timer for NUD.  Made the NUD timers 32 bits in
            milliseconds.

          o Made the random component for the time between subsequent
            Router Advertisements larger.  The minimum is now 1/3 of the
            maximum value resulting in the range between 0.5Tp and 1.5Tp
            suggested in [SYNC].

          o Require that host MUST maintain at least two default routers
            (rather than just 1)

          o Made the MTU option carry a 32 bit MTU (for jumbogram
            capable links)

          o Specified an infinity value (all one bits) for prefix
            Invalidation and Deprecation Lifetime.

          o Made NUD use timers to retransmit probes if the first probe
            is not answered.  This provides an hard upper bound on the



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            time it takes to detect an unreachable neighbor.

          o Specified that Neighbor Unreachability Detection applies to
            router-router communication unless there is some other
            mechanism which ensures two-way reachability between router
            neighbors.

          o Changed "ICMP unreachable error" to "ICMP unreachable
            indication" throughout, defined "indication" in the
            definition section explaining how errors are handled when
            they occur on the same node from where the packet
            originates.

          o Require that Router Advertisement, Router Solicitation, and
            Redirect messages are sent with a link-local source address
            for improved robustness.

          o Removed the use of Code 0 vs. 1 in all messages.  Added a
            "Router flag" field to the Neighbor Advertisement message as
            a replacement for the Code 0/1 distinction.

          o Added a Solicited flag to Neighbor Advertisements to make it
            possible to send unicast unsolicited advertisements without
            confusing the unreachability detection.

          o Specified the initial neighbor reachability state when
            creating Neighbor Cache entries.

          o Revised multihomed host section to better describe problems,
            without suggesting proper behavior.

          o Simplified the link-layer address encoding in the options by
            making it link specified.  Removed the address family field
            from the option.

          o Reduced the minimum allowed time for MaxRtrAdvInterval to 1
            second to make it more suitable for beaconing.

          o Relaxed the constraints on sending unsolicited Neighbor
            Advertisements.




     The changes incorporated in <draft-ietf-ipngwg-discovery-00.txt>
     compared to the previous version documented in:
          <draft-simpson-ipv6-discov-formats-02.txt>, and
          <draft-simpson-ipv6-discov-process-02.txt>



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     The changes agreed to at working group meetings at Xerox Parc and
     at Danvers IETF:

          o Renamed the Media-Access extension to be the Link-Layer
            Address extension.

          o Use of different extensions for addresses that refer to the
            sender of the packet and the receiver instead of using the
            Known-Identifier extension for both.

          o Changed the processing of General/Neighbor Solicitation in
            order to achieve 2 packet exchange just like ARP.

          o Removed the Node-Heard extension.


     Other changes:

          o Merged the processing and format documents into a single
            document with an extensive introduction to the protocol.

          o Aligned the document with [ADDRCONF].  In particular this
            implied the removal of the Change-Identifier extension.

          o Off-link prefixes are not advertized in Router
            Advertisements (no simple routing protocol).  This removes
            the need for a preference in the Prefix Information
            extension.

          o Specified a more detailed Neighbor Unreachability Detection
            algorithm (used to be called Dead Node Detection).

          o Removed the lifetime field from Neighbor Advertisements.
            The protocol uses Neighbor Unreachability Detection to time
            out state created by Neighbor Advertisements.

          o Removed the Maximum Receive Unit fields from packets since
            per-node MTU (or MRU) links do not work with multicast.
            Instead routers send an MTU extension in order to handle
            links that do not have a well-defined MTU.

          o Changed alignment mechanisms for extensions.  All extensions
            are a multiple of 8 octets.  Thus there is no longer a need
            for pad extensions.

          o Added support for anycast addresses.

          o Removed the ability to redirect prefixes to simplify host



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            processing.

          o Removed lingering mobility support (Mobility-Support
            extension and Remote Redirect message.)

          o All messages have separate ICMP types.  Redirect type is now
            in the error range (<128) and the others in the information
            range (>==128)

          o Moved fixed-length fields that are always present in a
            particular type of packet into the fixed header.

          o Renamed "General" Solicitation/Advertisement to "Neighbor"
            Solicitation/Advertisement.

          o Changed the default Router Advertisement period from 30
            seconds to 600 seconds; same value as in RFC-1256.  This
            change is possible since Neighbor Unreachability Detection
            will detect unreachable routers and switch a reachable
            router independent of the frequency of the Router
            Advertisements.

          o Specified rules for when a node should generate ICMP address
            unreachable errors due to Address Resolution failures.



























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     OPEN ISSUES

      - Default timer values for NUD?  Some routers might not respond in
        a timely manner to solicitations when they are busy processing
        routing updates.  NUD as specified will give up after 3
        transmissions spaced 3 seconds apart thereby requiring that a
        router respond in 9 seconds.

      - Timer values and retransmissions for address resolution.  Is 3
        transmissions separated by 1 second sufficient or should the
        nodes retransmit for a longer time?

      - Will all links (including point-to-point links) provide a link-
        local address?

      - Should we remove the Redirected header option?  The redirect
        message contains all the needed information so the only use of
        the included header is potentially for trouble shooting and/or
        if implementations want to verify the content of the included
        packet as being a packet that was recently sent.

      - Allow for balanced load sharing between multiple default
        routers?  This would require that hosts somehow randomly select
        a routers from the default router list.  Do we want to require
        hosts to do that?

      - Should it be possible to disable the Neighbor Unreachability
        Detection mechanism?  Is it sufficient to set the Reachable
        Retrans timer to 2^32-1 milliseconds? (about 46 days)






















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